Well I have found a workaround for now. Hope to get something better, but don't have time to look anymore.
https://forums.modx.com/thread/70203/tip-enabling-html5-elements-in-tinymce
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I dont wanna bump old threads but I have another solution in revolution.
1. Open: core/components/tinymce/templates/script.tpl
2. Add your code after this:
Tiny.config = <?php echo $this->modx->toJSON($this->properties); ?>;
In my case I add'd this code
Tiny.config.valid_elements = "*[*]";
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I am now able to use...
<i class="fa fa-lock"> </i>
it doesn't display in the editor, but at least it doesn't get changed to <em>