The mentioned element is not allowed to appear in the context in which you’ve placed it; the other mentioned elements are the only ones that are both allowed there and can contain the element mentioned. This might mean that you need a containing element, or possibly that you’ve forgotten to close a previous element.
One possible cause for this message is that you have attempted to put a block-level element (such as “
` is if you want to keep white-space, it's mostly used for blocks of code, to keep the line-breaks and the indentation. You normally wouldn't need ` ` either, I never need them anyway. If you need space between blocks of text consider using paragraphs instead and use css to control the space between them (with margin, there should be some by default if you aren't using a reset).
Most modern websites are html5 now yeah. I still like xhtml though, luckily in html5 you can stick to xhtml things like selfclosing tags. I don’t like parts of the spec where it’s okay to not close paragraphs or list items. But I like the clean doctype (one I can remember and not having to copy paste every time), the short charset meta, the short link and script tags and a whole lot of other things.