I always wondered why there is no underline ability?
Not even using HTML tags
Is this by design for all SE sites, or just a handful (or just AE?)
Please could we have an underline feature added?
The flavor of Markdown that Stack Exchange uses doesn't support this.
I would be against it in any case. Underlines are very commonly understood to indicate links when on web sites. You mess with this very basic and well-understood UI element at your peril. (I can't tell you how many times I've been on some web site and tried to click underlined text and nothing happened.)
It's been brought up before and unless you can devise a compelling new argument it's not going to be implemented.