You can use native HTML5 field validation
like e-mail validation (fiddle):
<input type="text" title="email" pattern="[^@]+@[^@]+\.[a-zA-Z]{2,6}" />
For your specific case you can use regexp like pattern="[a-zA-Z]*"
(fiddle)
When you submit form it became highlighted with red border to show you validation error. In different browser it will behave slightly different.
I don't think there is standard way to override every default styling, however there are browser specific ways to do this (here).
For some style you can have css hooks in place for changes see here
Edit: updated fiddle.