The tag allows front-end validation based on the xml in validation.xml.
For example the code:

generates the client side JavaScript for the form "logonForm" as defined in the validation.xml file.
The <html:javascript> when added in the JSP file generates the client side validation script.

Following tag displays all the errors: <html:errors/>

The Validator Framework uses two XML configuration files
1) validator-rules.xml and
2) validation.xml.
The validator-rules.xml defines the standard validation routines.
These are reusable and used in validation.xml to define the form specific validations.
The validation.xml defines the validations applied to a form bean.

The "Struts Framework" provides the functionality to validate the form data.

It can be used to validate the data in the user's browser as well as on the server side.

Struts Framework checks the JavaScript code and it can be used to validate the form data on the client browser.

Server side validation of form data can be accomplished by subclassing your "form" Bean with DynaValidatorForm class.

The "Validator" framework was developed by David Winterfeldt as a third-party "add-on" to Struts.

Now the Validator framework is part of the "Jakarta Commons" project and it can be used with or without Struts.

The Validator framework comes integrated with the Struts Framework and can be used without any making any additional settings.

An "ActionForm" is a JavaBean that extends org.apache.struts.action.ActionForm. ActionForm maintains the session state for web application and the "ActionForm" object is automatically populated on the server side with data entered from a form on the client side.