gitiles Markdown rendering fails to preserve <dfn> tags

[bcmills]

Via https://crbug.com/gerrit/12377:

Full URL exhibiting the problem:
https://go.googlesource.com/proposal/+/refs/changes/80/220080/3/design/36460-lazy-module-loading.md

Expected behavior:
<dfn> element present in the original Markdown source should be preserved, per https://spec.commonmark.org/0.29/#raw-html.

Observed behavior:
<dfn> elements are stripped out of the Markdown text rendered by Gitiles.