Symfony2 HTML in the trans twig filter
Try it with the twig raw filter:
{{ '<h3>foo</h3>' | trans | raw }}However, do not use the raw filter if you are processing any user input! It allows for cross-site-scripting attacks, according to the creators of Symfony. See this similar question for a secure but more tedious alternative.
Holding HTML stuff in translations is wrong, because translators usually break it. But if you really need it:
{% trans %}<h3>foo</h3>{% endtrans %}https://github.com/symfony/symfony/issues/2713#issuecomment-12510417