Note that those aren't placeholders. What you have discovered is the huge amount of iframes that pages uses to embed third party websites, usually ads, and where you blocked their domains through your "hosts" file (hosts, dns, is in essence the same) you found a normal load error page instead.
Besides create your custom CSS file (I can't provide my own rules that are way specific to my country/visited sites), you could just change the behaviour of third party frames by changing the value of the preference:
permissions.default.subdocument
There are a few macro helpers around the forum that may help you accomplish it.
One is this:
http://kmeleonbrowser.org/forum/read.php?9,135558,135558#msg-135558