[css-fonts-4] Font loading behavior when the descriptor is modified via CSSOM

[frivoal]

Issue first reported and discussed here: tabatkins/specs#69

(part of #1357)

Agenda+ since implementations are about to ship.

[frivoal]

cc @KenjiBaheux, since you were involved in the original issue.

[litherum]

Right now we have way more questions than proposals, and designing by committee is a great way to waste everyone's time. Removing Agenda+ until we have something more definite.

[KenjiBaheux]

Sorry, I missed the cc notification and wasn't aware that this was on the agenda at one point.

Here is what I think make sense and would help solving use cases from interested parties (e.g. crbug.com/762957).

When font display is changed, the UA considers the active period:

• block period
• swap period
• failure period

A font-display change can only affect the active period in the following manner:

• block => block
• block => swap
• block => failure
• failure => swap
• swap => failure

In order words, a block period can be extended (e.g. from optional and fallback's 100ms to block's 3s) but not introduced (e.g. starting from font-display: swap to font-display: block). This helps avoid hiding text that was already shown.

The timeout is only reset when the web font download is restarted or resumed. For instance, starting with font-display: optional a UA may decide to not download the font at all. A few seconds later, if the developer changes to font-display: fallback, the UA will start the web font download and start a swap period of 3s.

What do you think?

[KenjiBaheux]

cc/ @irori