Testing for Valid Code

The easiest way is to write valid code, either by-hand or using HTML-Tidy, as a stand alone application or as part of HTML-Kit.
Most commercial web coding applications do not produce valid code.
Several do have extra programs to help with validity.

The easiest way is to write valid code, either by-hand or using HTML-Tidy, as a stand alone application or as part of HTML-Kit.