Update: Looks like this is fixed in the latest CCleaner version 3.06.
About two weeks ago I ran CCleaner as part of my weekly maintenance, I updated it but failed to read that Steam had been added to the list of programs it can work on. After a few hours, I noticed that webpages viewed through the Steam client weren’t loading properly (stylesheets were missing or broken). I didn’t do anything about this until today when I found a quick-fix for it on Steam forums.
1. Exit Steam
2. Navigate to: C:\Program Files (x86)\Steam\config
3. Delete the “htmlcache” and “overlayhtmlcache” folders
4. Start Steam
I can confirm all is well now. I don’t know if CCleaner has been fixed or if it’s meant to do that but for now I’m just going to delete the cache folders manually.