Today, I found a very simple inconsistency between browsers, causing a major PITA for me. Assume this simple code:
<!DOCTYPE html>
<html>
<body>
<span style="font-size:50px;">LINE 1<br/></span>
<span style="font-size:14px;">line 2</span>
</body>
</html>
Which will look like this in most browsers:
But in internet explorer and edge, it will look like this
Interestingly enough, if you remove the DOCTYPE tag, both browsers will do the same. But I still don't understand why this happens.
Problems like this make it really fun to develop a website editor.
The behavior with DOCTYPE is probably even more miraculous, as like that there is neither public nor system identifier, so some browser-specific default might apply, and without the DOCTYPE another one might, so even when there are consistent parsing rules for a particular HTML version the DOCTYPE might select different versions maybe.
Probably not too helpful as quite vague, but maybe it helps.