Ok, another thing to try that I just noticed. Right click on a HTM or HTML file (whichever was affected) and choose Properties. What does the "Opens with:" line say?
(This is a wag but I noticed my PNG files don't have icons (show default executable icon), nor a default application, but when they are clicked they open up in the default picture viewer)
Otherwise, you can try deleting the HTM or HTML filetypes all together and just re-enter them. You'll have to manually add in the actions so write the stuff down first. A reboot between removing and adding probably wouldn't hurt either.