Why can HTML ignore Th attribute of Thmeleaf?Which HTML specification is it?

Tutorial:Using Thymeleaf(ja)

The quote below states that HTML ignores (the browser?).

Quote:

Most processors in standard directions are "attribute processors."Attribute processors allow you to view XHTML/HTML5 template files correctly in your browser even before they are processed.This is simply because the attribute is simply ignored.For example, a JSP using a tag library might not be able to display directly in your browser:
<form:inputText name="userName" value="${user.name}"/>
This is how the Thymeleaf Standard Dialect delivers similar features:
<input type="text" name="userName" value="James Carrot" th:value="${user.name}"/>
In addition to being able to display correctly in your browser, you can also specify the value attribute (optional) (the "James Carrot" part in this case).This value is displayed when the prototype is statically opened in the browser, and replaced by an evaluation value of ${user.name} when Thymeleaf processes the template.

I'm not familiar with writing th:value using colons for attributes, but what specifications of HTML ignore this?