Why does <u></u> not work?

Checking my site … It shows up in the HTML of the page, but it’s not translated on my theme. I’d say it’s your theme. It may be standard HTML, by the way, but most people associate underline with links.

If it’s not in your theme, it may be your browser. Either way, WordPress is, correctly, leaving the HTML alone.

Not true – the visual editor is replacing all <u> with <span style=”text-decoration:underline”>. It occurs when switching between HTML / visual editor, so presumably part of the editor js code.

This is undesirable, as it makes it impossible to style underlines (for example switching with a border-bottom).

Of well known note, switching back and forth between Visual and HTML editors causes weirdness.

I never use the visual editor. However <span style="text-decoration:underline"></span> works fine on my site too so … while it’s exceptionally annoying, it’s technically doing what you asked for: A bog standard underline.

My suggestion: If you really need full and absolute control of design like that, use the HTML editor.

I know, I know, the Visual is easier for a lot of people, my family included. I just never use it because it’s not as robust and makes me fit into this little, predefined, box. I write, then I go back and apply styles, then I preview, then I post. The repeat as needed happens a lot before I post.