Iframe lazy-loading is official

[javiercasares]

In #3 we talked about giving support to iframes and others, but it wasn't official yet.

Now it's official
whatwg/html#5579

Supported in Chromium (Chrome / Edge) Firefox and Safari.

+info (Google): https://web.dev/iframe-lazy-loading/

[1jj]

Supported in Chromium (Chrome / Edge) Firefox and Safari.

Not true.
https://bugzilla.mozilla.org/show_bug.cgi?id=1622090
https://bugs.webkit.org/show_bug.cgi?id=196698