An entity reference was found in the document, but there is no reference by that name defined. Often this is caused by misspelling the reference name, unencoded ampersands, or by leaving off the trailing semicolon (;). The most common cause of this error is unencoded ampersands in URLs as described by the WDG in “Ampersands in URLs”.
Entity references start with an ampersand (&) and end with a semicolon (;). If you want to use a literal ampersand in your document you must encode it as “&” (even inside URLs!). Be careful to end entity references with a semicolon or your entity reference may get interpreted in connection with the following text. Also keep in mind that named entity references are case-sensitive; &Aelig; and æ are different characters.
If this error appears in some markup generated by PHP’s session handling code, this article has explanations and solutions to your problem.
Note that in most documents, errors related to entity references will trigger up to 5 separate messages from the Validator. Usually these will all disappear when the original problem is fixed.
Also XHTML5 (and the earlier versions of XHTML) don’t work with Internet Explorer 8 or earlier - the page just gets offered for download instead of displaying in those browsers.
One place where XHTML5 is less strict than HTML5 is that XHTML5 doesn’t need the doctype as it only has one display mode.
I believe that the point that @Mittineague was making is that the HTML validator does not recognize &sensor and &libraries as valid markup; therefore, it flags them as errors. The validator flags known errors and assumes unrecognized code is an error. It doesn’t recognize google’s “creative” code. With experience, you learn to tell the difference between legitimate errors and “erroneous errors”.
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