Have been working on a site and all of a sudden, this morning, it was broken.
AAAAHH!!!! Run around in a panick.
Then I noticed it was only broken in Chrome Canary (their dev browser) and the break was very specific:
<legend> tags were having a height applied to them, perhaps inherited from their parent
<fieldset> tag? The numbers didn’t add up, so it was hard to tell. Also, it only caused obvious display problems if the legends were being displayed as their own block instead of part of the field set border, but on inspection the blocks were still sized incorrectly behind the scenes.