This is defined by the javascript jQuery validator.
There is also an issue there :
- The Arrow is missing and it is confusing
- When error, the arrow is still missing (on red box)
2 solutions :
- Ask the developper to fix that (I cannot do it for the moment)
- Bypass it and show the informations somewhere else in the field
TODO (second solution) :
- edit classes/jformfield.php
- Add a special treatment that receive the validatorMsg params, and design a HTML / CSS inside.
Placing it there, this call will be instancied for each fields
This is the reason why every single field type added in your component MUST be inherited from your class file (no direct use of native Joomla fields). Well, you can do what you want, but you will not be able to use the validators (JS) features.