VE has a tooltip for inserting big text that's still using <big>...</big> tags (see here). This is not supported in HTML5, hence it should be replaced with appropriate CSS (probably it should use <span style="font-size:larger">...</span>, but I don't know whether that's going to fit every use case).
|Resolved||None||T2209 [DO NOT USE] HTML validity (tracking)|
|Open||None||T26529 Incrementally remove support for HTML elements removed from or deprecated in HTML5|
|Open||None||T265287 Stop using <big> tags for bigger text in tooltips|
There's a long discussion about this on T40487 (it started before VE was a thing). It's not clear if it's desirable to remove or replace the syntax (even if it's obsolete HTML, we might want to continue supporting it by adding our own CSS or parser handling for the tag).
Whatever we do about this, the situation in VE should be kept synchronized with the old wikitext editor. The old editor still has the corresponding button right now, so we shouldn't remove ours.