[madmulita]

At least the header has something in it, the white margins are completely useless but eat a lot of real state.

[UncleEntity]

Yes, the ability to "tweet" and "share". Neither of which I ever do so it's just a bunch of wasted space.

My favorite is the headers that mess up scrolling with the space key -- scroll down then immediately scroll up because the header is covering part of the content.

And don't even get me started on sites (looking at you Medium) that have both headers and footers...

[exergy]

ublock's "content rule" feature gets a lot of use out of me for this exact reason.