Onclick attribute looks not allowed in Plone 5.2

Hi

I have already add onclick into the Custom attributes list in @@filter-controlpanel
I add following code in text/x-web-textile format

  <a class="prev" onclick="plusSlides(-1)" >❮</a>
  <a class="next" onclick="plusSlides(1)" >❯</a>

After click save to visit the sources code of the page, it disappear:

<a class="prev">❮</a>
<a class="next">❯</a>

Anyone could help me to fix the problem?

Thanks.
Alex