NOTE: For typical usage of CSS HTML Validator, you should not have to change the active categories or otherwise use the Configuration Editor.
CSS HTML Validator comes with an included configuration file that supports many different 'categories' of elements. Support for HTML 5.2, HTML 5.1, HTML5, HTML 4.01, HTML 4.0, XHTML 1.0, XHTML 1.1, HTML Microdata, Netscape extensions (legacy), Microsoft Internet Explorer extensions (legacy), WML, XHTML and more is already included. Note that some categories such as WML are only supported by the standard and higher editions.
An important characteristic of the included configuration file is that the categories are built upon one another. This means that if you want to validate against HTML 3.2, then the HTML 2.0 category, in addition to the HTML 3.2 category must be active. To validate against HTML 4.0 (or XHTML), the HTML 2.0, HTML 3.2, and HTML 4.0 categories must all be active (HTML 4.0 cannot be the only active category). This is because the HTML 4.0 category requires the HTML 3.2 category as its "foundation", and the HTML 3.2 category requires the HTML 2.0 category as its "foundation". Similarly, supporting HTML5 requires HTML 4.01 and all of HTML 4.01's underlying categories to also be active.
The default is for most categories to be active. If needed, categories that are not active may automatically be made active by a tag name program when a document is checked. Therefore, depending on how a specific category works, a non-active category may still allow syntax checking of that category even if it is inactive.
•The Dynamic HTML and Scripting & Applets categories should normally be made active or inactive together. Disabling these categories is considered very unusual and is not recommended.
•Use the "cseignore" element to surround markup that you want HTML Validator to ignore during a validation. Example: <cseignore>...ignore this...</cseignore>. See also: How to Ignore Parts of a Document
•This category should always be active (enabled). Disabling this category is considered very unusual and is not recommended.
•The Miscellaneous category contains these attributes: "ms_positioning", "msimagelist", "naturalsizeflag", "nof", "nosave", "webbot-action", "xmnls:o", and "xmnls:v".
•This category is disabled (not active) by default because the elements and attributes that it contains are now considered so obsolete that they are not even worth recognizing.
•The Miscellaneous Extensions category contains non-standard extensions that are often supported by multiple vendors, like the "incremental" attribute that is supported by both WebKit (Safari) and Blink (Chrome). (New 20170803)
•This category is normally enabled (active) but is automatically disabled for standards-compliant checks.
•The XML 1.0 category contains the "xml:lang" attribute.