XHTML is essentially a stricter form of HTML, but not quite. For the most part, you can satisfy the requirements of both standards at the same time. However, when it comes to closing tags, the two standards are incompatible. For example, the line break tag in HTML is
<br> but in XHTML is
<br/>. Most browsers will tolerate the unnecessary backslash before the closing tag in HTML, especially if you put a space before it. But it’s not strictly correct.
So is this just a pedantic point of markup language grammar? Chris Maunder says an error with closing tags caused Google to stop indexing his website. He had XHTML-style end tags but had set his DOCTYPE to HTML.
I’ve also heard of browsers refusing to render a page at all because it had DOCTYPE set to XHTML but contained an HTML entity not supported in XHTML. I believe the person reporting this said that he had run the XHTML page through a validator that failed to find the error. Unfortunately I’ve forgotten where I saw this. Does anyone know about this?