The OP of that question is Manishearth, from the Rust core team!

I suppose he was working on Servo when looking into this sort of question.

Sorry, I can't find where it discusses why it may have looked fine in the past, can you point me to it more precisely?

Ah, found it in one of the earlier HN threads: https://news.ycombinator.com/item?id=7352391

The image linked there doesn’t exist

The explanation is there however, see the next comment.

It looks okay on internet explorer with compatibility view on.

But, as has been asked many times, WHY?

Because in old IE, <h3> was defined with an absolute font size in the built-in style sheet, in newer browsers it is defined as a relative size. Since we have nested <h3>'s they get increasingly bigger in newer browsers, but in old IE they would all have the same size.

It's because some browsers close the currently open heading tag when they encounter a new one (since these tags can't be nested) and others do not. Since the text size is applied as an H3 style and is relative to the parent, the nesting of these tags (or not) determines whether the effect of the style is cumulative.

If I understand the other arguments correctly, IE also saw them as nested, but defined the text size of h[1-6] elements as absolute, not relative.