Chrome Version: 70.0.3521.2 (Official Build) dev (64-bit)
OS: Ubuntu 18.04 Linux

What steps will reproduce the problem?
(1) Visit https://jsfiddle.net/xs6rwvmz/ , or just view this HTML:

<fieldset style="writing-mode: vertical-rl">
  <legend>vertical-rl</legend>
</fieldset>

What is the expected result?
For this vertical-rl fieldset: the fieldset-border's "legend gap" should be on the right side (the block-start side) of the fieldset, in the spot where the legend text renders.

What happens instead?
The legend text renders on the right side, but the gap for it is on the left side.