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

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?