The issue probably comes from the fact that web browsers try to render HTML even if it’s not perfect. HTML isn’t super strict, so browsers will still display pages with small mistakes. There was a push to make HTML stricter with XHTML, which enforced rules like case-sensitive elements and closing tags, kind of like XML. But it didn’t really stick. Browsers had a hard time with those stricter rules, so HTML’s more relaxed approach stuck around. For some time I really tried to use XHTML when createing weh pages, but then I asked my self why all of the trouble when browsers don't follow the standards.