Pre-transform that changes deprecated border attribute to CSS.
Occasionally, a certain attribute will need to be removed and popped onto another value. Instead of creating a complex return syntax for HTMLPurifier_AttrDef, we just pass the whole attribute array to a specialized object and have that do the special work. That is the family of HTMLPurifier_AttrTransform.
An attribute transformation can be assigned to run before or after HTMLPurifier_AttrDef validation. See HTMLPurifier_HTMLDefinition for more details.
transform(array $attr, \HTMLPurifier_Config $config, \HTMLPurifier_Context $context) : array
Abstract: makes changes to the attributes dependent on multiple values.
| array | $attr | Assoc array of attributes, usually from HTMLPurifier_Token_Tag::$attr |
| \HTMLPurifier_Config | $config | Mandatory HTMLPurifier_Config object. |
| \HTMLPurifier_Context | $context | Mandatory HTMLPurifier_Context object |