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In my Wordpress post I click on edit post and skip over to the html editor tab and click on it. I have a pre on that page and when i give it a rel and publish its all fine but when I skip back over to visual editor it just takes out the rel attribute.

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Why do you want to add an invalid attribute to pre? rel is for links. – toscho Apr 8 '12 at 22:23
up vote 2 down vote accepted
function extend_tinymce( $values ) {
    $elements = 'pre[id|class|title|rel]';
    if ( isset( $values['extended_valid_elements'] ) ) {
        $values['extended_valid_elements'] .= ',' . $elements;
    } else {
        $values['extended_valid_elements'] = $elements;
    return $values;
add_filter('tiny_mce_before_init', 'extend_tinymce');

The above snippet is what I use for example, of course you can extend this beyond id, class, title, rel, to things such as height, width, style, name, etc...

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