How was Microsoft able to customize the tab title style? [closed]

Looking at new Microsoft page for Windows 10 , at least on Ubuntu 14 / Chrome 43 , I noticed that the size of the source of the flap was bigger than the source of the other open flaps. In Firefox 38, this does not happen.

"Win features [...]" appears larger than other titles.

Can anyone else reproduce this behavior? Would it be a bug?

I took a look at the DOM properties of the page, but I could not understand why this happened.