Add support for loading="lazy" to iframes e.g. YouTube embeds and others

[Kirby]

As per https://xenforo.com/community/threads/a-few-more-things.183091/ XenForo does support lazy loading

However, IFrames (such as YouTube embeds) seem to be missing loading="lazy"

 

[ivp]

WordPress just implemented Lazy-loading iframes, it would be nice to see it in XenForo too:

Lazy-loading iframes in 5.7

In WordPress 5.7, iframes will be lazy-loaded by default, using the browser-level loading attribute. This will bring to iframes the same performance benefits which WordPress 5.5 introduced by lazy-…

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