HTML notebook outputs such as pandas DataFrames are missing from the served page and appear only after JavaScript runs

[mmcky]

Found while re-measuring #206 for the #147 gate. It is filed without a parent: readers with JavaScript do see the tables, so no lecture page visibly breaks at cutover.

Problem

On a MyST lecture page, a notebook output is left out of the HTML the site serves when its data holds anything besides plain text or a PNG, GIF, JPEG or BMP image. Pandas DataFrames are the common case. The output's container is served empty, and the table is built in the reader's browser after the page's JavaScript loads. The Sphinx lecture sites serve the same tables inside the page itself.

Measured on 2026-09-11 on the /pandas page of lecture-python-programming's jb2 preview (theme v2.7.0), using Chromium at 1440px:

	Sphinx: https://python-programming.quantecon.org/pandas.html	MyST: jb2 preview /pandas
DataFrame outputs	33 tables	34 outputs with text/html + text/plain in the page data
table.dataframe in the served HTML (JavaScript off)	33	0
table.dataframe with JavaScript on	33	33, about 0.5 s after DOMContentLoaded

Who notices:

• No JavaScript run: readers without JavaScript, and any tool that reads the served HTML without running scripts, such as crawlers that don't execute JavaScript, link previews, archivers and reader views.
• Everyone on first load: the tables appear after the text around them, which shifts the page.

Why it happens

• Most outputs skip server rendering. @myst-theme/jupyter 1.3.0 renders an output directly only when it is a stream or error, or when every one of its MIME types is text/plain, PNG, GIF, JPEG or BMP (DIRECT_OUTPUT_TYPES and DIRECT_MIME_TYPES, dist/output.js lines 16-23; isOutputSafe, lines 24-39). Every other output goes to JupyterOutput (line 74).
• JupyterOutput only runs in the browser. It builds the DOM with thebe-core's rendermime registry inside useEffect (dist/jupyter.js lines 9-35). Upstream calls this interim: "FUTURE: we'll be rendering AST outputs directly in future" (dist/output.js line 60).
• The theme adds nothing here. Its only output override wraps stderr streams and hands every other output to upstream (app/renderers.tsx L79-L101).
• The QuantEcon mystmd fork converts HTML outputs to AST, but only for PDF. reduceOutputs runs text/html outputs through htmlTransform (transforms/outputs.ts L247-L317). It runs only when simplifyFigures is set (process/mdast.ts L543-L547), and only the tex and typst builds set it; the site build does not.

Options

Option	Where	What	Trade-offs
A. Server-render static HTML outputs	This theme's output override	For an output whose richest type is text/html with no scripts, render a sanitised copy of that HTML on the server and keep it through hydration. Interactive outputs (scripts, plotly, widgets) stay with JupyterOutput	Ships with the theme alone, and works on stock mystmd too. Needs an HTML sanitiser in the bundle, a rule for what counts as static, and handling for the <style scoped> blocks pandas emits
B. Convert HTML outputs to AST at build time	QuantEcon/mystmd, site build	Reuse reduceOutputs' htmlTransform path, so static text/html outputs reach the page as ordinary content that myst-to-react renders on the server	DataFrames become ordinary tables (theme-styled and searchable). Conversion is lossy for complex HTML, and it is fork-only until upstreamed, so stock-mystmd builds keep today's behaviour
C. Render outputs from AST upstream	jupyter-book/myst-theme	The direction upstream's own comment points to	The best long-term home; no timeline

Acceptance criteria

• On a lecture-python-programming build, /pandas shows its DataFrame tables with JavaScript disabled.
• With JavaScript enabled, each table renders once, with no duplicate after hydration, and interactive outputs such as plotly figures and widgets still work.
• No unsanitised author HTML reaches the server-rendered page.
• A visual-test fixture stores a text/html DataFrame output, and a test asserts the table is present in the served HTML.

Context

• Source: found while re-measuring Notebook output plots sit flush left in the content column, where the Sphinx lecture sites centre them #206 (output image alignment) on the jb2 preview. The 2026-09-11 parity study's row F067 ("Notebook cell input/output presentation", partial) notes that the book theme renders DataFrames through myst-nb, but it did not compare the served HTML.
• Alignment is fine: once rendered, DataFrames sit left-aligned on both sites, so Notebook output plots sit flush left in the content column, where the Sphinx lecture sites centre them #206 needs no table change.
• Related: Book-theme parity cutover — tracking #147 (cutover gate; this issue is not on it) and Notebook output plots sit flush left in the content column, where the Sphinx lecture sites centre them #206.
• Upstream: no existing issue found in jupyter-book/myst-theme (searched 2026-09-11 for server-side or static rendering of HTML outputs).