Define the "memory cache"

[annevk]

Aside from the preload cache, browsers have another cache for certain resources. The semantics differ across browsers and can impact whether service workers end up seeing a request, for instance. Ideally this would be better defined.

#590 tracked some of this discussion previously.

[noamr]

Note: the memory cache is defined for images, here. However, it is implemented for other resources types as well (styles/fonts/...)

[noamr]

Some existing preloaded-related bugs that arose from lack of defined memory cache:
https://bugzilla.mozilla.org/show_bug.cgi?id=1743764
https://bugs.webkit.org/show_bug.cgi?id=233854

[domenic]

whatwg/html#6110 is the HTML-side issue for this.

[youennf]

Service worker and memory cache relationship is also something that would be good to describe.