Question

Without any specific regularity my vim displays underlines on the place of tabs (see below).

Sometimes it also happens to the text: I type and it’s underlined.

What could be a reason?

Answer 1

This is likely due to the fact that you are editing an html file and the text near the underline is inside of an <a> tag.

To disable this you can add let html_no_rendering=1 to your ~/.vimrc. This setting will, however, also disable bold and italic styling for html files.

If you wish to only disable the underlining, see :help html.vim. There it gives you instructions on what highlight groups you need to redefine without underline.