Use <pre class=idl> for IDL blocks

[foolip]

A friendly request. In https://github.com/mdittmer/web-apis @mdittmer is working on tooling for extracting the IDL from specs linked to from Chromium's source code, for the purpose of comparing the IDLs. This will hopefully be a powerful tool for avoiding accidental differences.

In order to avoid having to use a real browser to scrape the IDL, it's useful if a <pre class=idl> block is used. This is one spec that Mark found relying on ReSpec's <dl class=pre> feature.

Note that this is already a ReSpec warning: "Defining WebIDL in dl elements is deprecated. Please use Contiguous IDL instead: https://github.com/w3c/respec/wiki/User's-Guide#webidl-support"

[marcoscaceres]

Will work on this...