features.md

Features

Loading and rendering

• Lazy ZIP: reads only the central directory, inflates each part on demand (native DecompressionStream, 0 dependencies)
• HTTP Range: open a URL → fetch only the bytes needed (256 KB block cache); server without Range support → automatic full-download fallback
• Streaming XML parser runs in a Web Worker → UI never blocks
• Progressive render:
  • DOCX: blocks (paragraph/table) stream in → offscreen height measurement in time slices → pagination → only visible pages render
  • XLSX: rows stream in, 2D virtualized grid (only cells in viewport render), freeze panes, merge, number format, sheet tabs
  • PPTX: slides parsed on demand when scrolled into view, virtualized thumbnails

Format support

Format	Supported	Not yet
DOCX	inherited styles, run formatting, lists/numbering, tables (merge, border, shading, table style), images, hyperlinks, header/footer, PAGE field, text boxes, page/section breaks, text columns, tracked changes	footnote body, charts, exact float wrapping, paragraphs split across pages
XLSX	shared strings, number/date formats, font/fill/border/align, merge, freeze, hidden columns/rows, wrap, formula calculation (135 functions, recalculated on edit), multiple sheets	charts, conditional formatting, images
PPTX	master/layout/theme, placeholder inheritance, ~50 preset shapes, custGeom, images (crop), tables + table style, groups, bullet/autonum, autofit	charts (placeholder), SmartArt, animations, video playback

Password-protected (encrypted) files are not supported.

Word, Excel and PowerPoint 97-2003, RTF and OpenDocument

These files open editable: the worker converts them in memory to the Office Open XML file the views show, and Save writes that file (report.doc is saved as report.docx, budget.ods as budget.xlsx; the original is never overwritten). The bar shows a DOC / XLS / PPT / RTF / ODT / ODS / ODP badge, Document details says "Word 97-2003 document, opened as Word document". Nothing leaves the browser and no library is added: every format is read by the library's own code in the worker.

File	Opens as
.doc, .dot	document (Word 97-2003)
.xls, .xlt	workbook (Excel 97-2003, also Excel 5.0 / 95 and workbook streams outside a compound file)
.ppt, .pot, .pps	presentation (PowerPoint 97-2003)
.rtf	document
.odt, .ott, .fodt	document (OpenDocument text, template, flat XML)
.ods, .ots, .fods	workbook (OpenDocument spreadsheet, template, flat XML)
.odp, .otp, .fodp	presentation (OpenDocument presentation, template, flat XML)

The format comes from the file's bytes (compound file, {\rtf, OpenDocument mimetype, flat XML), so a .doc that is really RTF, or a file without a name, opens too.

Word 97-2003 (.doc)

• Converted: paragraph and character styles (built-in ones get Word's names: Heading 1, Title…), fonts, sizes, bold / italic / underline kinds / strikethrough, caps, colors, highlight, shading, borders, super/subscript, spacing, alignment, indents, line spacing, tabs, keep with next, page breaks; bullet and numbered lists (9 levels); tables (merged cells, borders, shading, padding, row heights, header rows, nested tables); sections (page size, orientation, margins, columns, section breaks), headers and footers (first page, even pages), footnotes and endnotes, comments, bookmarks, tracked insertions / deletions; fields (links, PAGE / NUMPAGES, the rest with their result), inline and floating pictures, text boxes and shapes, groups, WordArt text
• Not yet: Word 6.0 / 95 and older files (an error says so), table styles, drop caps, formatting revisions; EMF / WMF pictures are kept in the file (Word shows them) and shown as a placeholder

Excel 97-2003 (.xls)

• Converted: every sheet (hidden ones stay hidden), values of every type, the shared string table, fonts, fills, borders, alignment, wrap, indent, rotation, number formats (built-in and custom), Excel 2007+ colors and themes, merged cells, column widths and row heights, hidden rows / columns, outline levels, frozen panes, gridlines, zoom, the active sheet; formulas (every token, shared and array formulas, names, 3-D references) with their last values; defined names, print areas, page setup, headers and footers, hyperlinks, cell comments, tab colors; Excel 5.0 / 95 files
• Not yet: pictures, charts, conditional formats, data validation; chart sheets open empty; formulas pointing at other workbooks keep their values only

PowerPoint 97-2003 (.ppt)

• Converted: slide size, masters and title masters (backgrounds, shapes, placeholders, text styles, color schemes), slides with their own backgrounds and color schemes, hidden slides, footers / slide numbers / dates; about 190 shape types, freeforms, fills (solid, gradient, picture, texture, pattern), lines (dash, arrowheads), shadows, rotation and flips, groups, tables (as grouped cells); text with bullets (also numbered), fonts, sizes, colors, alignment, spacing, links, slide number and date fields; pictures; speaker notes and comments (saved in the file)
• Not yet: PowerPoint 95 and older files (an error says so), animations and transitions, sounds and movies, charts and embedded objects (their preview picture is shown); EMF / WMF pictures are kept in the file and shown as a placeholder

RTF

• Converted: every code page (also Shift-JIS, GBK, Big5, Korean), \u Unicode text, Symbol / Wingdings characters; paragraph and character styles (built-in ones get Word's names), fonts, sizes, bold / italic / underline kinds / strikethrough, caps, colors, highlight, shading, borders, super/subscript, spacing, alignment, indents, line spacing, tabs, frames; bullet and numbered lists (also the older WordPad / Word 95 kind); tables (merged cells, borders, shading, padding, header rows, row heights, nested tables); sections (page size, orientation, margins, columns, breaks, page numbering, first page), headers and footers (first page, left / right pages), footnotes and endnotes, bookmarks, links, PAGE / NUMPAGES fields (the rest with their result), tracked insertions / deletions; PNG / JPEG / GIF / BMP pictures (size, scale, crop), text boxes and shapes in groups
• Not yet: comments, equations, drawing objects and lines, drop caps, line numbering, page borders; EMF / WMF pictures and OLE objects (their preview picture is shown when it is a bitmap)

OpenDocument text (.odt)

• Converted: named styles (LibreOffice's built-in ones get Word's names), automatic styles as direct formatting, every character and paragraph property the DOCX view shows (fonts for each script, sizes, colors, underline kinds, strike, highlight, super/subscript, caps, letter spacing, alignment, margins, line height, keep together / with next, breaks, borders, backgrounds, tab stops), numbered headings, bullet / numbered / picture lists (9 levels, continued numbering), tables (spans, header rows, borders, backgrounds, padding, nested tables), pictures (inline and anchored) and text boxes, links, bookmarks, footnotes and endnotes, comments, page styles as sections (size, orientation, margins, columns, page numbering), headers and footers (left / first page), page number and count fields, tables of contents (as their text)
• Not yet: tracked changes (shown accepted), lines and connectors, objects with only a StarView preview, drop caps, line numbering, picture cropping

OpenDocument spreadsheet (.ods)

• Converted: every sheet (hidden, tab color, right to left), values (numbers, percentages, currencies, dates, times, booleans, errors), formulas translated to Excel's syntax with their last values, number formats (also conditional), cell styles (font, fill, borders, alignment, wrap, rotation, indent), merged cells, column widths and row heights, hidden rows / columns, outline groups, frozen panes, gridlines, zoom, the active sheet, named ranges, print ranges, page setup with headers and footers, hyperlinks, comments. The empty rows and columns LibreOffice writes to the end of a sheet (a million rows) are not expanded
• Not yet: pictures and charts, conditional formatting, data validation, rich text inside a cell

OpenDocument presentation (.odp)

• Converted: slide size, master pages as slide masters (theme colors and fonts, backgrounds, drawings, placeholders, title / outline text styles), slides with their own backgrounds, hidden slides, date / footer / slide number; text with lists, bullets and numbering, fonts, sizes, colors, alignment, spacing, links, fields; preset and custom shapes (draw:enhanced-geometry drawn as custom geometry), lines and connectors with arrowheads, fills (solid, gradient, bitmap, hatch), shadows, rotation, skew and flips; pictures (cropped), groups, tables (cell styles, merged cells), speaker notes and comments (saved in the file), text that shrinks to fit
• Not yet: charts and embedded objects without a picture (a labeled box), 3D scenes, Fontwork effects, animations and transitions

Not supported: password-protected files, Word 6.0 / 95, Excel 2.x-4.0 and PowerPoint 95 files, OpenOffice.org 1.x files (.sxw, .sxc, .sxi): each gives an error saying so.

Apple Pages, Keynote and Numbers (view only)

Files from Pages 5, Keynote 6 and Numbers 3 (2013) and later open read-only: the worker converts a .pages document to DOCX, a .key presentation to PPTX and a .numbers spreadsheet to XLSX, in memory, and the matching view shows it. Find, print, word count, slideshow, sheet tabs and Download as work as usual; Download as Word / PowerPoint / Excel saves the converted file. The bar shows a PAGES / KEYNOTE / NUMBERS badge and no Edit button, even with editable. Single files and zipped package folders (Report.pages/… inside a ZIP) both open.

Pages

• Converted: paragraph and character styles (named styles become Word styles), fonts, sizes, bold/italic/underline/strikethrough, colors, highlights, super/subscript, caps, alignment, indents, spacing, page breaks, bullet and numbered lists (levels), links, inline images, text boxes and shapes with a fill, tables (merged cells, header rows, cell fills, banded rows, lines, text/number/date cells, rich text cells), header and footer (left, center and right parts, page number), footnotes (listed after the text), page size and margins
• Not yet: charts, equations, comments, tracked changes, exact positions of floating objects (placed in the text flow: those on the first page before the body, the others after it; on page layout documents, page by page), image cropping, several sections with different headers

Keynote

• Converted: slide size and order, skipped slides (hidden), master slide objects (drawn under the slide's), slide backgrounds (color, gradient, image), title / body / slide number placeholders, text boxes (text styles, bullet and numbered lists, links), shapes (rectangles, rounded rectangles, polygons, arrows, stars, custom paths and curves, fill and line), images cropped by their mask, movie poster frames, groups, rotation and flips, tables (merged cells, header rows, cell fills, banded rows, lines)
• Not yet: charts (a placeholder box), builds and transitions, speaker notes, shadows and reflections, text that shrinks to fit

Numbers

• Converted: every table becomes a worksheet (named after its sheet, plus the table's name when a sheet holds several), hidden sheets stay hidden; values keep their type: numbers, currencies, percentages, scientific and dates with the matching Excel number format (Numbers date patterns translated), booleans; durations, fractions, numbers in other bases, checkboxes, star ratings and pop-up menu choices as the text Numbers shows; rich text cells as their text; cell look (font, size, bold/italic/underline/strikethrough, color, alignment, vertical alignment, wrap, fill, banded rows, lines), column widths, row heights (grown to fit wrapped text), merged cells, hidden rows / columns, frozen header rows / columns
• Not yet: formulas (cells hold the values Numbers computed), charts, images and text boxes placed on sheets, custom formats (shown with the General format), accounting-style currency, the document locale's decimal / thousands separators, conditional highlighting, comments

Cell values in Pages and Keynote tables use the same formats.

Not supported: password-protected files and the iWork '09 and older XML format: each gives an error saying so.

CSV, text, Markdown, HTML and SVG

A src (file, Blob, buffer or URL) that is not an Office package opens too, converted in the worker and editable:

File	Opens as
.csv, .tsv / .tab	workbook, one sheet named after the file; the rows stream in as the file is read (a million rows open like a large workbook; past 12 million cells the rows are read from the file by pages as they are shown); numbers and booleans typed, codes like 007 kept as text
.txt, .text, .log	document, one paragraph per line
.md, .markdown	document: headings, lists, bold / italic / strike, links, code, quotes, tables (as tab-separated lines)
.html, .htm, .xhtml	document: headings, paragraphs, lists, inline formatting and links; scripts and styles are dropped
.svg	the picture alone (no page), scaled down to fit the view, at its own size (width / height in any unit, else the viewBox) otherwise; the file is kept as it is, given the SVG namespace when it lacks one; Download as Word gives a document holding the picture
.svgz, .csv.gz, .md.gz…	the file inside: a gzip-compressed plain file is unpacked whole in the worker (native DecompressionStream), the name inside telling the format

• Format: the extension (of name, the File or the URL), else the MIME type (Blob type, Content-Type), else the content (SVG, HTML, delimited rows, or text). A gzip file (.svgz, .csv.gz) is unpacked first and the name inside decides. A file named .docx / .xlsx / … is always read as a package, so an HTML error page served for a missing file still fails
• CSV: comma, semicolon (then 12,5 is a number) or tab, detected from the first line; quoted fields, newlines inside quotes
• Streamed: the text goes from the file straight into the sheet, in chunks, in the worker (never a string of the whole file, never a workbook built first): the first rows show at once and the rest streams in like a large workbook's. Column widths come from the first 5 000 rows. Saving writes every row from the sheet
• Paged: past 12 million cells (about 300 MB in Chrome) the rows stay in the file: the loader notes where each page of 1 024 rows starts and goes on counting, and a page is read when its rows are shown, edited, searched, exported or saved, the least recently shown pages making room (pages with edits stay). So there is no row limit. In such a sheet rows and columns cannot be inserted or deleted, and an edit or a formula may not need more rows than fit in memory (=SUM(A:A) over 100 million rows is refused with a message). residentCells in the open parameters sets the budget
• Excel's limit: a sheet of more than 1,048,576 rows cannot be saved as a workbook: the bar says so, saving fails with a message, and Download as CSV / TSV gives the rows (written in the worker, page by page)
• Encoding: byte order mark (UTF-8, UTF-16), the declared charset, UTF-8, else Windows-1252 (Excel's CSV on Windows)
• The bar shows a CSV / TSV / TXT / MD / HTML / SVG badge
• Saving: a Markdown, HTML or SVG file is saved as itself (.md / .html / .svg, also what onSave receives). Markdown and HTML are built again from the document when it was edited there: headings, lists (nested), bold / italic / underline / strike, links, code, colors, tables, pictures (embedded as data URLs; a diagram block becomes its drawing); the text reads with its tracked changes accepted. An unedited file is saved as it is. CSV / TSV and text are saved as a workbook / document (.xlsx / .docx); Download as Word / CSV / text / Markdown gives the other formats

Diagrams: Mermaid, PlantUML, Graphviz

Diagram sources open as a document holding the drawing (an SVG picture, editable around it, saved as .docx). The drawing is made in the worker by a built-in renderer: no server, no network request, no third-party library.

File	Draws
.mmd, .mermaid	Mermaid: flowchart / graph, sequence, class, state, ER, pie, Gantt, mind map
.puml, .plantuml, .pu, .iuml, .wsd	PlantUML: sequence, class / object, use case, component / deployment, state, activity, mind map, WBS, @startdot
.dot, .gv	Graphviz DOT: graph / digraph, clusters, default attributes, edge chains, rank=same, record and HTML labels (text)

• In other files: ```mermaid, ```plantuml / ```puml, ```dot / ```graphviz code blocks in Markdown, and <pre class="mermaid">, <div class="mermaid">, <pre><code class="language-…"> blocks in HTML are drawn in place; other code blocks stay code
• Format: the extension, else the MIME type (text/vnd.graphviz…), else the content: @startuml, digraph {, a Mermaid header (graph TD, sequenceDiagram…), also for a Blob or URL served as text/plain. A .dot file that is a Word 97 template (binary) is not read as a graph
• Layout: layered, like Mermaid's and Graphviz's dot (ranks, fewer crossings, straight long edges, clusters kept apart); TB / LR / BT / RL; node shapes, edge labels, arrowheads (UML, crow's foot), colors from classDef / style / DOT attributes
• Page: the drawing is scaled down to fit the page; a wide one gets a landscape page. Several diagrams in one file (PlantUML blocks, DOT graphs) are drawn one after another
• Safe: the text is escaped, colors are kept only when they are plain colors, and the picture has no scripts, links or external references
• Code side by side: the Code button (Markdown, HTML, SVG and diagram files) shows the source next to the document, with line numbers and syntax colors; editing it rebuilds the document as you type (the page keeps its scroll position). Meanwhile the document is a preview (the source is what is edited). For Markdown and HTML the pane shows the source built from the document when that was edited last, so the two never disagree; a diagram's document can be edited around the drawing, but that is not in its source (the pane says so), and Save writes the Word file. The source pane copies or downloads the source. In a narrow viewer the source goes above the document; the divider can be dragged (or moved with the arrow keys)
• Not drawn: other Mermaid kinds (git graph, journey, timeline, quadrant, C4, Sankey…), other PlantUML kinds (Gantt, JSON / YAML, salt, timing, network), the Graphviz engines other than dot (neato, circo… are laid out as dot), HTML tables in DOT labels (their text is shown). A diagram that cannot be drawn shows why, followed by its source

Editing

DOCX

• Typing: type/delete/Enter, paste (keeps formatting when pasting from Word/Google Docs/web), IME input (e.g. Vietnamese)
• Text formatting: B/I/U/S, superscript/subscript, font, size, color, highlight, paint format
• Styles: Normal, Title, Subtitle, Heading 1–6 always available (created in the file when missing; Ctrl/⌘+Alt+0…6), update a style to match the selection (right-click → Update style to match: font, size, color, B/I/U/S, alignment and spacing go into the style, every paragraph of it follows; written to that file's own styles only), document outline panel
• Paragraphs: alignment, line spacing, bullet/number list, indent, page break
• Images: insert (file picker/paste/drag & drop), click to select with 8 resize handles, original size, fit width, replace, alt text, download
• Tables: insert, add/delete rows & columns, delete table, Tab between cells; select cells by dragging across them, merge / unmerge (across and down), cell color, cell borders (all / outer / inner / none / one side, color, line style), vertical alignment, distribute columns, drag a column edge to resize
• Page setup: paper size (A4, Letter, Legal, A3, A5, B5), portrait / landscape, margins (cm, presets), 1–3 text columns with spacing; applies to the whole document
• Headers & footers: header, footer and first-page parts (double-click the page's header / footer area, or the toolbar), alignment, bold, size, page number / page count fields with presets ("Page 1 of 5", "Trang 1/5"), different first page
• Tracked changes: insertions and deletions from Word shown in their author's color and kept on save; next / previous change, accept / reject (the change at the caret, the selection, or all); Track changes records your own edits as suggestions (typing, deleting, pasting, replacing)
• Other: select all (Ctrl/⌘+A), link (dialog), special characters, right-click context menu (clipboard, link, table, image, insert, styles), find & replace, word count, undo/redo

XLSX

• Cells: edit, range selection, select all (Ctrl/⌘+A), copy/cut/paste, fill down/right, sort, find & replace (match case, entire cell, in formulas)
• Formulas: auto-calculated (135 functions), function name & argument hints while typing, click/drag cells (incl. other sheets, arrow keys) to insert references, colored references, auto-close parentheses, functions menu (SUM/AVERAGE/COUNT/MAX/MIN)
• Formatting: paint format, currency/percent/decimal buttons + 20 number formats, font, size, B/I/U/S, text/fill color, borders (8 kinds, color, line style), merge all/horizontally/vertically, horizontal & vertical alignment, wrap, text rotation, clear formatting
• Rows & columns: insert/delete/hide/unhide, drag to resize column width/row height, freeze, gridlines
• Sheets: add/rename/delete/reorder
• Other: context menu, undo/redo

PPTX

• Text: font, size, B/I/U/S, color, highlight, superscript/subscript, alignment, vertical alignment, line spacing, bullet/numbered list, indent level, clear formatting
• Shapes: select/drag/resize/delete (arrow keys to nudge), fill, border color/weight/dash, arrange (front/forward/backward/back), rotate 90°/flip, align on slide, duplicate (⌘D), undo/redo
• Insert: 30 shapes, text box, image
• Slides: background, add/duplicate/delete/move/hide, slideshow from current slide or beginning
• Other: context menus on shapes/slides/thumbnails

Toolbar

Single row, buttons that don't fit collapse into ⋮; custom-drawn dropdowns (font/style preview, number formats with examples, ↑↓/Enter/Esc/type-ahead keys, never steals focus from the document); font size box − 11 + accepts direct input; cell borders picked from an icon grid; color pickers have a palette plus a Custom row (+ opens the system color picker, recent custom colors are remembered across sessions).

Dark mode

theme="dark", or "auto" to follow the OS / browser setting (prefers-color-scheme), darkens the chrome: bars, toolbar, menus, dialogs, sheet headers and tabs, thumbnails, popovers, scrollbars. Pages, slides and cells keep the colors the document gives them, like paper in a dark PDF viewer. Everything is driven by CSS variables on .de-root, so a host can also tune the palette.

Comments

• Where: DOCX — the selected text (or the word at the caret); XLSX — a cell (one thread per cell, as in Excel: commenting on a cell that has one answers it); PPTX — next to the selected shape, or where the slide is right-clicked
• Start one: Ctrl/⌘+Alt+M, right-click → Comment, or Comments → New comment
• Threads: replies, resolve / reopen (answering a resolved thread reopens it), edit and delete your own comments, deleting a thread's first comment deletes the thread
• @mentions: type @ to pick someone from the host's list (comments.mentions); the host hears who was mentioned (comments.onEvent) to notify them
• Panel: threads in document order with what they point at (the commented text, Sheet1!B3, Slide 2), resolved ones on demand; picking a thread shows its place, clicking commented text / a marked cell / a pin on a slide shows its thread
• In the document: commented text is highlighted (stronger for the current thread), cells get a mark in their top right corner, slides a pin with the author's initials; resolved threads are not shown
• DOCX ranges follow the text: typing inside a commented range extends it, typing right after it does not; splitting a paragraph keeps both parts covered; undo takes a new comment away (redo brings it back); a thread whose text is deleted leaves the file
• XLSX: threads move with inserted / deleted rows and columns and go with a deleted row, column or sheet (undo brings them back)
• PPTX: a slide's threads stay with it (not copied by Duplicate slide, deleted with the slide)
• Shared rooms: comments are shared operations like edits — every editor sees them live, newcomers get them from the file or the history
• Read-only while viewing (default: comments can be written while editing, comments.canComment overrides)

Stored the Office way, so Word, Excel and PowerPoint show them:

Format	Parts	Notes
DOCX	comments.xml, commentsExtended.xml (threads, resolved), commentsIds.xml, commentsExtensible.xml, people.xml	ranges as commentRangeStart / commentRangeEnd + reference; untouched comments keep their XML (rich text); comments in headers / footnotes are kept
XLSX	threadedComments/*.xml, persons/person.xml, plus the legacy comments*.xml + VML Excel keeps next to them	notes (legacy comments) are read as one-comment threads; a note nobody answered or changed stays a note
PPTX	modern comments comments/modernComment_*.xml, authors.xml	legacy comments are read as one-comment threads and written back as modern comments

Comment text is plain (lines kept); formatting inside comments read from a file is kept while the comment is not edited. Ids are durable: a comment keeps its id after saving and reopening (DOCX durable ids, XLSX / PPTX GUIDs).

File menu

• New: blank document, spreadsheet or presentation (16:9, Office theme, title / content / blank layouts)
• Open (Ctrl/⌘+O): .docx .xlsx .pptx open as they are; .doc .xls .ppt .rtf and OpenDocument .odt .ods .odp are converted (editable, saved as .docx / .xlsx / .pptx); .csv / .tsv become a workbook (numbers and booleans typed, codes like 007 kept as text); .txt / .md / .html become a document (headings, lists, bold / italic / strike, links, code); .svg / .mmd / .puml / .dot become a document with the picture
• Download as: the native format, PDF, web page (.html, images embedded), plain text / Markdown (DOCX), CSV / TSV of the current sheet (XLSX), slide text with notes (PPTX)
• Import CSV into new sheet (XLSX): adds a sheet after the current one with the file's rows, streamed in the same way (in a shared session the rows go to everyone as cell edits)
• Save (Ctrl/⌘+S), Rename (shown name and the name used for saving), Document details (file, core properties, page / word / slide counts)
• Host entries (fileMenuItems: Share, Version history…) and Close (onClose)
• PDF / Print (Ctrl/⌘+P): rendered off-screen and sent to the browser print dialog (choose "Save as PDF"): DOCX pages as paginated on screen, PPTX one slide per page, XLSX current sheet fitted to the paper width

Saving

• DOCX: edited paragraphs are regenerated from the original XML (keeps pPr, rPr, images, bookmarks, comments); untouched paragraphs keep original bytes
• XLSX: only <row>s containing edited cells are rewritten (streamed, never loads the whole sheet into a DOM); new strings written as inline strings; fullCalcOnLoad set so Excel recalculates formulas
• PPTX: only a:p of edited shapes are regenerated
• Unchanged ZIP entries are copied as-is (no recompression)

Operation	File	Time
Type 1 char (browser)	DOCX 2073 pages	~7 ms
Save after edit	DOCX 20k paragraphs	~0.5 s
Save after editing 2 cells	XLSX 300k rows (25 MB)	~3 s
Open (rows streamed in)	CSV 1M rows × 10 cols (99 MB)	~2.4 s
Open (rows paged, ~1 GB of memory)	CSV 25M rows × 10 cols (2.7 GB)	~14 s
Open (rows paged, ~1 GB of memory)	CSV 100M rows × 10 cols (10.6 GB)	~50 s
Open (rows paged, <1 GB of memory)	CSV 500M rows × 10 cols (55 GB)	~4.5 min
Save (every row written)	CSV 1M rows → XLSX (56 MB)	~13 s

Limitations

• Formulas: no defined names, structured refs (Table1[Col]), 3-D refs
• DOCX: text boxes and nested tables are read-only; paragraphs with fields spanning multiple paragraphs (TOC) are read-only; headers / footers are edited as text lines (pictures, tables and shapes in them are replaced when edited, with a warning); a header taller than the top margin overlaps the text; tracked changes don't cover formatting changes, paragraph marks or pictures; page setup applies to the whole document (not per section)
• XLSX: no chart/pivot/conditional format/data validation/filter editing; inserting/deleting rows & columns doesn't shift conditional formats/data validation; a sheet read by pages (a CSV past 12 million cells) takes no row / column insert or delete, and its formulas only the rows that fit in memory
• PPTX: can't rotate shapes with the mouse yet, can't edit text inside tables yet, no table insert, no animations/transitions; slide list changes (add/delete/move) are not undoable
• Comments: plain text only; DOCX comments can't cover text in more than one table cell or across a table; comments in headers, footers and footnotes are kept but not shown; XLSX comments don't move with sorted rows; PPTX comments anchored to a shape are shown at the shape's position when the file is opened and keep that anchor until moved