CSS HTML Validator checks documents based on W3C and WHATWG standards but also based on what works with real-world browsers. We recommended that you use the default settings. However, you may want to check your documents in a similar manner as the W3C validator does.
There are a few things you can do to check closer to W3C standards.
NOTE: CSS HTML Validator does not use the exact same validation engine as the W3C validator, so the validation results will not be exactly like the W3C validator. Use nsgmls (see below) to get the closest results to the W3C validator.
You can quickly do a standards-compliant check by choosing Validate->Standards-compliant check. This validation mode will automatically disable non-standard categories of tags. With a standards-compliant check, non-standard tags (like the old "blink" and "marquee" tags) and attributes will generate error messages. Note that even with this option, the results may not be exactly the same as the W3C validator. CSS HTML Validator will still check for many issues that the W3C validator cannot find.
Furthermore, when doing a standards-compliant check, the CSS checker will upgrade non-standard CSS property messages to errors. For example, messages generated by flag 600 will be upgraded to errors.
Using the Configuration Editor in the pro or enterprise edition, the non-standard categories can be deactivated. This will help to provide more standards-compliant results for all validations.
To do this, go to Options->Configuration Editor and the Categories tab. Remove from the Active categories list box the non-standard categories like the Microsoft IE Extension categories and the Netscape Extension categories. Save the configuration file to a different filename other than the default and use the new configuration file for future validations. NOTE: If you save the configuration file to the default filename, then it may be overwritten the next time CSS HTML Validator is updated.
You can use nsgmls to validate many pre-HTML5 documents by choosing Validate->Nsgmls messages only. This will use an SGML parser and validator (nsgmls) to validate the document according to a DTD. This is very similar to what the W3C validator does for pre-HTML5 documents. If this option is selected, then the CSS HTML Validator validation engine will not be used.
NOTE: Using only nsgmls and not the CSS HTML Validator validation engine will greatly limit the number of problems that may be found in your documents. We recommended that you always use the CSS HTML Validator validation engine even if you use nsgmls.
If you want to perform "Nsgmls messages only" validations by default, then uncheck flag 115.
If you want the validation results to include both the CSS HTML Validator validation engine messages and nsgmls messages, then enable nsgmls messages in the Nsgmls tab of the Validator Engine Options.
See also the DTD validation information.