The following notes and warnings highlight missing or conflicting information which caused the validator to perform some guesswork prior to validation, or other things affecting the output below. If the guess or fallback is incorrect, it could make validation results entirely incoherent. It is highly recommended to check these potential issues, and, if necessary, fix them and re-validate the document.
No Character encoding declared at document level
No character encoding information was found within the document, either in an HTML meta
element or an XML declaration. It is often recommended to declare the character encoding in the document itself, especially if there is a chance that the document will be read from or saved to disk, CD, etc.
See this tutorial on character encoding for techniques and explanations.
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
A DOCTYPE declares the version of the language used, as well as what the root (top) element of your document will be. For example, if the top element of your document is <html>, the DOCTYPE declaration will look like: "<!DOCTYPE html".
In most cases, it is safer not to type or edit the DOCTYPE declaration at all, and preferable to let a tool include it, or copy and paste it from a trusted list of DTDs.
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
The element named above was found in a context where it is not allowed. This could mean that you have incorrectly nested elements -- such as a "style" element in the "body" section instead of inside "head" -- or two elements that overlap (which is not allowed).
One common cause for this error is the use of XHTML syntax in HTML documents. Due to HTML's rules of implicitly closed elements, this error can create cascading effects. For instance, using XHTML's "self-closing" tags for "meta" and "link" in the "head" section of a HTML document may cause the parser to infer the end of the "head" section and the beginning of the "body" section (where "link" and "meta" are not allowed; hence the reported error).
</html>