Markdown

Hollo supports Markdown posts, including tables and ~~strikethrough~~. Your instance operator can enable additional syntax with MARKDOWN_EXTENSIONS. Extensions apply when creating or editing local posts. Profile biographies and fields keep their existing syntax.

Enable extensions

Set the extensions you want in .env, then restart Hollo:

MARKDOWN_EXTENSIONS=footnote,ruby,math

The available names are footnote, deflist, sub, sup, mark, ruby, ins, abbr, tasklist, and math. An unset or empty value disables them all. Spaces around names and duplicate names are ignored; an unknown name prevents startup.

Configuration changes leave stored source and HTML unchanged and send no updates. Explicit edits use the current setting; editing after disabling an extension can reveal its original syntax.

Syntax

Extension	Source	Meaning
footnote	Text[^1] and [^1]: Note	Numbered footnote
deflist	A term followed by : Definition on the next line	Definition list
sub	H~2~O	Subscript
sup	x^2^	Superscript
mark	==highlight==	Highlighted passage
ruby	{漢字:한자}	Ruby reading annotation
ins	++inserted++	Added text
abbr	*[HTML]: Hyper Text Markup Language	Abbreviation definition
tasklist	- [x] Done and - [ ] Pending	Static task states
math	$x^2$ and $$x^2$$	Inline and display mathematics

For example, with the corresponding extensions enabled:

A statement[^1]. HTML has elements.

[^1]: Supporting detail.

Term
: Definition

*[HTML]: Hyper Text Markup Language

- [x] Written
- [ ] Reviewed

Footnotes include visible numbers and absolute links to the original post. Definition lists include visible punctuation to separate terms from their definitions. Ruby includes fallback parentheses for readers that remove tags; Gukhanmun preserves explicit ruby instead of adding nested readings.

Abbreviations are case-sensitive and use word boundaries. HTML5, html, and HTML은 do not match HTML. The first matching use in ordinary text includes the visible expansion, such as “HTML (Hyper Text Markup Language)”. Link labels, URLs, mentions, hashtags, code, and math are excluded. Duplicate definitions use the first definition; expansions are plain text.

Tasks show static [x] or [ ] text; [X] also means completed. Follow markers with whitespace or a line ending. Edit the post to change state. - [x]: note can be a Markdown link definition instead of a task.

Escaping and ambiguous text

Use code spans or a backslash before a delimiter to write literal syntax. With sup or sub, paired carets or tildes in emoticons and ranges can become formatting: write \^_\^ or 1\~2,3\~4 to preserve them. Spaces inside subscripts and superscripts must be escaped, as in x^a\ b^.

With footnote, ^[note] is an inline footnote; with sup, ^[note]^ is superscript. Explain essential exponents in words too, such as “x to the power a+b”, since receiving software may flatten them.

Mathematics

Inline $...$ cannot cross a line. No whitespace may follow its opening $ or precede its closing $; the closing $ cannot be followed by a digit. Thus $5 and $20 and $30 remain ordinary currency text. Escape a literal dollar as \$, or use a code span.

Display math starts with $$ at the beginning of a line, allowing indentation. Only whitespace may follow the closing $$ on its line. $$x^2$$ can occupy one line; display math in the middle of a sentence is unsupported.

The inline expression is $x^2$.

$$
\frac{a}{b}
$$

Math uses MathML with a TeX annotation, following the approach described in FEP-dc88. Invalid expressions, unsafe commands, or rendering limits show escaped literal source. Each expression has separate macros with limits on expansion and size.

Limits are 8,192 UTF-16 code units per expression, 64 expressions per post, 256 KiB output per expression, and 1 MiB total math output. When the post exceeds 65,536 UTF-16 code units, only math parsing is disabled: ordinary Markdown and mention, hashtag, and link processing continue. Dollar-delimited text then follows ordinary Markdown rules, rather than being protected math.

Reading on other servers

Receiving software can remove subscripts, superscripts, highlights, insertion marks, and abbreviation tooltips. Write important corrections and definitions in visible text. Ruby support in Mastodon and Misskey depends on the content and receiving version.

Mastodon can expose math annotations as dollar-delimited TeX text; this is not a guarantee of typeset mathematics. Misskey can concatenate presentation text with the TeX annotation. Display varies between servers and clients. Explain essential formulas in ordinary language too.

Actual server-to-server acceptance tests covered Mastodon 4.7.3 and Misskey 2026.10.0. Both stored all ten examples. Mastodon applied edits; Misskey 2026.10.0 ignored Note updates and retained the original content. The recorded results include received HTML/MFM and rendering losses. Browser, client, and accessibility checks remain pending.