It's annoying :(
<rdar://problem/19149826>
If I do: shell> document.body.classList.add("tEsT") Then hover <body> in the inspector the page overlay label does say "body.tEsT". Are you seeing otherwise?
Seems to be a quirks mode thing. No doctype (quirks), we canonicalize lowercase class name: data:text/html,<h1%20class="tEsT">Test With doctype, we show class name as is: data:text/html,<!doctype%20html><h1%20class="tEsT">Test