Support text highlights with <mark>

[willkg]

Problem

• read drops highlights. Confluence's editor can give a run of text a background colour. It is stored as <span style="background-color: rgb(…)"> and becomes an ADF backgroundColor mark. read writes only the text, so a read → edit → update cycle removes every highlight from the page. (Verified 2026-09-26 on a scratch page.)
• There is no way to write a highlight in Markdown. ==text== is not GFM and stays literal. A raw <mark> passes through, but Confluence drops the tag and keeps only the text. Only a raw <span style="background-color: …"> works, and nobody would guess that.

What Confluence did with each spelling (storage read back, then ADF):

written	stored	takes effect (ADF)
<span style="background-color: #fff0b3;">	kept, rewritten as rgb(255,240,179)	yes, backgroundColor
the same in rgb()	kept	yes
<mark>	tag dropped, text kept	no
<span data-highlight-colour="…"> (the table-cell attribute)	attribute dropped	no
<span style="color: …"> (text colour, for comparison)	kept	yes, textColor

Proposal

Use <mark>, which GitHub renders:

Markdown	published as
<mark>some text</mark>	<span style="background-color: #fff0b3;">some text</span> (a default colour)
<mark style="background-color: #ffbbbb;">some text</mark>	<span style="background-color: #ffbbbb;">some text</span>

read writes a highlighted span back as <mark> (the default colour) or <mark style="background-color: …"> (any other colour), so the page survives a round trip. Markdown inside a <mark> is still converted.

The attribute is background-color, not color, because in CSS color is the text colour.

Open questions

1. Default colour. #fff0b3 is the table-cell palette's yellow. The editor's text-highlight palette may differ; read back a highlight made in the editor to find out.
2. Names or only hex? Cell colours accept swatch names. A name is not valid in a CSS background-color, so either hex only, or names in another attribute.
3. An unrecognised colour. Publish with a warning, as an unknown cell colour does, and have check report it?
4. Text colour. read probably drops <span style="color: …"> the same way. Handle it here, or in a separate issue?