Sadly there's a bug in [NSMutableAttributedString readFromUrl:options:DocumentAttributes:] if you enable WebKit that causes SubEthaEdit to crash on load of certain HTML files. Were aware of this now, and will fix this with the next minor update. Very sorry for the inconvenience.
In the meantime: html-files with an <html> or <!DOCTYPE tag but no <body> trigger the crash. So in the meantime you can add a <body> tag to your documents which will also make your html more valid. Nevertheless its an crashing bug that we didn't catch, and there may be other combinations that trigger it. Again, sorry for that.