From https://www.htmlvalidator.com/help.php? ... 9.0300+Pro[65] This document's DOCTYPE is not a recognized DOCTYPE and may not be valid. Note that most DOCTYPEs are treated as being case-sensitive. For HTML 4.01 Transitional documents, use "<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">". The unrecognized DOCTYPE is: "<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "https://www.w3.org/TR/html4/loose.dtd">". See the description of flag 65 in the documentation for more information and the list of recognized DOCTYPEs.
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Life is full of little things. I assume the Documentation got updated to https:// and the software flag did not.HTML 4.01 Transitional <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "https://www.w3.org/TR/html4/loose.dtd">
CSS HTML Validator Professional 2019 (64-bit) Editor v19.0300(Pro) Engine DLL v19.0300(Pro)