I'm unable to get #13, `<mark>`, to work, as in the tags are appearing in line with the text:

To be <mark>continued…</mark>

I'm really liking these tips, but can someone test if all/most of them are working?

Lazy loading images can be an issue when your internet connectivity is slow or erratic. You might find yourself waiting for the image to load and then seeing it fail.

Either way, good alt-text for images is a must.