htmlpurifier/library/HTMLPurifier/AttrTransform/Background.php

<?php

/**
 * Pre-transform that changes proprietary background attribute to CSS.
 */
class HTMLPurifier_AttrTransform_Background extends HTMLPurifier_AttrTransform {

    public function transform($attr, $config, $context) {

        if (!isset($attr['background'])) return $attr;

        $background = $this->confiscateAttr($attr, 'background');
        // some validation should happen here

        $this->prependCSS($attr, "background-image:url($background);");

        return $attr;

    }

}