valign attribute / vertical-align on table cells is not supported

[KaiserKarel]

Related to #507 (same real-world emails). Table cells have no vertical alignment support: the servo-mode stylo build doesn't include the vertical-align longhand at all, and blitz doesn't map the valign presentational attribute either. Cell content always flows from the top.

Real-world impact

HTML-email bar charts (Sentry weekly reports and similar) build columns as <td valign="bottom" style="height:110px"> containing a shorter bar table — without valign, every bar hangs from the top and the chart renders upside down.

Minimal repro

<table style="width:100px"><tr>
  <td valign="bottom" style="height:110px;font-size:0">
    <table style="width:100%"><tr>
      <td height="55" style="background-color:#7553FF;font-size:0">&#160;</td>
    </tr></table>
  </td>
</tr></table>

Expected: purple block in the bottom half of the 110px column. Actual: top half.

Approach we took

Since taffy 0.12's block layout supports align-content, we map the cell's valign attribute onto the cell's inner block style (bottom → AlignContent::END, middle/center → CENTER, top → START) at taffy-style build time: fifteenlabs/blitz@244abec. That sidesteps the missing stylo longhand; a fuller fix would add vertical-align to the stylo property set. Happy to upstream the attribute-level version if useful.

[nicoburns]

Stylo does have vertical-align. It's just a shorthand that decomposes into the alignment-baseline, baseline-source and baseline-shift longhands.

(see: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/vertical-align)

That being said, I think what you have here is the correct fix anyway.