I just noticed that HTML files with CSS transforms on elements show with the CSS transforms within the thumbnails on Nautilus. So what rendering engine does Nautilus use to generate the thumbnails for HTML files?
The thumbnailing programs are installed separately except for text/plain and are specified in gconf here:
This system allows you install different programs to do the rendering of different kinds of images. My system doesn't have any html preview/thumnails and gconf reflects that. So I wonder what gnome packages you have installed that provide that functionality.
If it doesn't work not open gconf-editor go to /desktop/gnome/thumbnailers/text@html and change the command
gnome-web-photo --mode=thumbnail --timeout=16 --size=%s %u %o
/usr/bin/gnome-web-photo --mode=thumbnail --timeout=16 --size=%s %u %o
Normally you can reload the view to get the thumbnails. If you want to recreate all icons see here: Refresh thumbnails in nautilus