You can control the format — font, size, and so on — of error message text, or you can substitute a glyph for error text. Validation controls support the same properties as other controls for specifying their look.
To format error messages
Note Some of the settings you make with these properties may not be supported in all browsers.
You can provide formatting within an error message by using HTML elements in the text for the ErrorMessage and Text properties. For example, to apply italics to only one word in the error message, you can surround the word with <I> and </I> tags.
Note To display reserved HTML characters within the error message text, encode them. For example, to display "<", use the character sequence <
.
If you are displaying validation errors in a summary, you can specify that a glyph appear in place of the individual validation control.
To display glyphs next to input controls
Note The glyph is displayed in place of the validation control. It's up to you to lay out the validation control in such a way that the glyph displays properly.
Validation Controls | Introduction to Validating User Input in Web Forms |