Continuing on from a previous article I wrote earlier in the year... 'Using a html form as the model', this post describes using validation.
With html 5 comes a whole bunch of validation you can use in your forms. This makes a html form even more useful as a schema than the html 4 forms. Since you can use types like email, telephone, time, date, and such. You can also use things like minimum, and maximum length. As well as the ever useful 'required' field.
You can read about html 5 forms here.