HTML: Guidance about lists of values in attributes

[LeaVerou]

This relates to #269.

For short lists of values, current practice is to use a space or comma separated attribute. Examples:

• Global class attribute (space separated)
• <iframe sandbox> (space separated)
• <input accept> (comma separated)

For larger lists of values, embedding the list in an attribute should be discouraged. Instead, current practice is to link to another element (e.g. <input list>) or to declare the list using child elements, if this is an essential part of the element (e.g. the list of <option> elements inside <select>).

[cynthia]

Similar to what #278, this probably should touch on how the attribute is exposed to the DOM, and also for APIs that accept sequences. (and how interop is expected to work - className ergonomics is probably and example that would work as an antipattern.)

[plinss]

See also #270

[LeaVerou]

Consensus from today's breakout:

• Shorter lists should be space-separated and exposed as DOMTokenList
• Consistency with other parts of the web platform trumps that (the <input accept> case)
• Longer lists should be separate elements (<datalist> or <source> elements)