"HTML attribute" as a term

[martinthomson]

On #485, a number of other alternatives were floated.

• DOM attribute
• content attribute
• element attribute
• markup attribute

The point being that in some of the cases, we are not talking exclusively about HTML, but also SVG, MathML, and other syntaxes.

Personally, I don't think that any of the above would be an improvement in the cases that I can see this being used, but we thought it would be worth considering this.

[annevk]

The only term with a defined meaning from these in the standards world is "content attribute". I guess you can define one of these as well, but generally I'd prefer we reuse the same terms. (Renaming "content attribute" is on the table too I guess, if people really dislike it.)

[LeaVerou]

I think using content attribute unless we actually only mean HTML makes a lot of sense.

[hober]

@matatk, @hadleybeeman, and I talked about this briefly during a breakout today, and we all agree to use content attribute.

Action on @matatk to write a PR.