Deprecate the font HTML tag

[zecakeh]

Rationale:

MSC4077 allows to deprecate HTML tags that are deprecated in the WHATWG standard without requiring an MSC, if they can be replaced by tags with the same feature.

font is deprecated and can be replaced by span with the data-mx-bg-color and data-mx-color attributes.

So, HTML like this:

<font data-mx-bg-color="#FF0000" color="#00FF00">Some green text on a red background</font>

Can also be written like this:

<span data-mx-bg-color="#FF0000" data-mx-color="#00FF00">Some green text on a red background</span>

Preview: https://pr1739--matrix-spec-previews.netlify.app

[tulir]

<font> is used more than <strike>. Strikethrough was always done using <del> in most clients, but the /rainbow command in the latest element web version outputs <font color="..."> tags. This also effectively reverts MSC2422

It might still be a good idea to remove it, but I think it'd be good to have a note saying it was previously allowed. Maybe mark it as deprecated somehow instead of removing entirely

[KitsuneRal]

A note saying that clients should stop using font and switch to span would be nice indeed. As for <font color=…>, I surmise sticking to one way of colouring characters is a good thing.

Outdated

[zecakeh]

I added an info box for the deprecation.

[tulir]

This looks fine now, but I'm not completely sure if it should require an MSC due to being used by many clients

[KitsuneRal]

LGTM. Given that the change in the clients is quite straightforward, I don't think we should hold back this change. After all, the whole HTML subset is recommended, not mandatory, and cases of similar minor non-compliance have happened and will happen. But I'd appreciate a third opinion from another client author, out of caution.

[turt2live]

I'm not a terribly active client dev myself these days, but the change seems reasonable given the context. Clients are asked to prefer span, but we aren't breaking anything.

Opening issues on a few clients would be appreciated, but not required. This might be an SCT responsibility to encourage the ecosystem to upgrade anyways.