[mishig25]

Problem

Math in docstring parameter and return descriptions renders as literal text instead of math. For example, on AnyFlowTransformer3DModel, the gate_value parameter shows the text {@html "<span class="\"katex\"> followed by raw KaTeX markup with broken attributes like class="\"mord" mathnormal\". The sample parameter on FlowMapEulerDiscreteScheduler is broken the same way. Both come from :math: in diffusers docstrings, which doc-builder converts to \\(…\\); $…$ breaks the same way.

Cause

renderKatex in kit/preprocessors/mdsvex/index.js wraps every rendered formula in a Svelte {@html "…"} tag. That's right for the doc body of a page, which is Svelte markup. But docstring.js renders each docstring section as a separate mdsvex fragment, and the result is an HTML string that Docstring.svelte displays with {@html description}. The nested {@html tag is never evaluated: the browser shows it as text and parses the JSON-escaped quotes inside it as attribute values.

Fix

• When the content starts with DOCSTRING_FRAGMENT_MARKER, renderKatex returns the plain KaTeX HTML. Page bodies still get {@html …}.
• docstring.js rewrites }} as }‌} in parameter descriptions. That made KaTeX fail on any }} in parameter math, e.g. $\frac{1}{\sqrt{r}}$, and the error stops the build. renderKatex now restores }} before rendering.

Verification

• Snapshot: a new mix_embeddings sample covers $…$ and :math: in parameter descriptions, the }} case, and math in a return description. No existing line of page.html changes, including the body math of legacy_function. With main, the sample renders the same broken markup as the diffusers page; on this branch it's KaTeX.
• Unit tests: two new tests in mdsvex/index.test.js, one for page-body math and one for fragment math. The fragment test fails on main.
• Page bodies: with main and with this branch, I ran the mdsvex preprocessor on transformers' perplexity.md, on a synthetic page with every math form ($…$, \\(…\\), $$…$$, braces, math in a table), and on the full snapshot page. The output is byte-identical.
• Round trip: parseRenderedParamsList re-serializes the KaTeX HTML. For \frac{1}{\sqrt{r}}, the result is identical to KaTeX's own output except that the zero-width space comes back as ​, which the browser decodes to the same character. The SVG of \sqrt is kept.
• Checks: npm test passes (12/12), as do pytest tests/test_docstring_snapshots.py, ruff and prettier. The 3 ESLint errors in mdsvex/index.js are already on main.

Once diffusers' docs rebuild, these blocks become translatable: #903 keeps them in English until then.

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