The DOCTYPE
you are trying is invalid, it should be <!DOCTYPE html>
which is a valid doctype
for HTML5 and as far as the custom attributes go, you need to use data-
prefix which will be considered as valid custom attribute.
<p data-custom-attribute="Value"></p>
[1] A custom data attribute is an attribute in no namespace whose name starts with the string
data-
, has at least one character after the hyphen, isXML
-compatible, and contains no uppercaseASCII
letters.
1. Reference