Why is letting HTML dangerous?

[Lea Verou]

Originally Posted by cheat-master30

1. They can use CSS and styling to disrupt the layout massively, or make parts of the login box or other features/links disappear from view.
2. As said, Javascript cookie stealing.
3. Javascript causes really annoying effects such as things flying around or maybe the page upside down/flipped.
4. Iframes to embed viruses and other malware.
5. Iframes or forms to embed fake forms for phishing purposes/stealing passwords, even making the fake form look part of the site.
6. Crashing the browser with an extremely large image.
7. Redirects to other, potentially dangerous/offensive pages.
8. Browser exploits.
9. Annoyances such as leaving tags open to turn everything bold under the empty tag or italic or underline etc...

So, they can't harm the whole site, just the current page?
If so, then these blogging sites are not doing anything dangerous, each blog is its blogger's responsibility...