BEM and the long class name
A naming convention is a bet that the next person reads class names before they read CSS, and the ugly double underscores are paying for something real, which is not having to know the whole stylesheet just to change one thing.
The 400 KB hero
Every conversation about page weight goes straight to minification. Meanwhile there is a 2 MB image on the page that nobody has opened since Photoshop.
abbr and the title attribute
The abbreviation element is one of about four places in HTML where the title attribute is the actual content rather than a decoration, and it is still unreachable for anybody not holding a mouse.
IE8 is a client, not a browser
Browser support looks like a technical decision and gets argued as one, but it's a commercial decision with a number attached, and the number is usually sitting in the analytics if you go and look for it.
float: left is a layout system
We are building every grid on the web out of a property invented to wrap text around a picture, and the pile of workarounds we've grown on top of it has stopped looking like workarounds.