htmlpurifier/library/HTMLPurifier/AttrTransform/Border.php

<?php

/**
 * Pre-transform that changes deprecated border attribute to CSS.
 */
class HTMLPurifier_AttrTransform_Border extends HTMLPurifier_AttrTransform {

    public function transform($attr, $config, $context) {
        if (!isset($attr['border'])) return $attr;
        $border_width = $this->confiscateAttr($attr, 'border');
        // some validation should happen here
        $this->prependCSS($attr, "border:{$border_width}px solid;");
        return $attr;
    }
    
}