How to reproduce :

• In the newsletter template, put that code :

  .newsletter-content h3 { clear:both; font-size:1.3em; }
  .newsletter-content .ametys-cms-content h3{ clear:none; }

  and take care of use the same classes in the content like :

  <div class="newsletter-content"><div class="ametys-cms-content news vevent wrapper"><h3>My title</h3></div></div>

• send the newsletter and view it in a webmail.
  > if you inspect the code, the priority rules applied on the H3 tag are reversed :

  <h3 style="clear:none;font-size:1.3em;clear:both">My title</h3>