One thing that might be added is a notice when you try to bring in HTML and it doesn't work to switch to HTML Tidy. You might offer it as a switch to HTML Tidy for input. I couldn't see anything wrong with the HTML, but with just pretty print on, it would not bring it in. Otherwise it worked great.
Including the ability to add files has eliminated my need for calibre, since Sigil will bring in a series of HTML files from the web, put them in sections and even bring in the illustrations. WOW!
One other suggestions for add files is a third button to stop the add process altogether. I already had the files through importing the HTML, but I selected all of them and said to add. I had to either add or cancel 50 times!