Actually Thunderbird developers are planning to go full on html+js as well:<p><a href="https:&#x2F;&#x2F;mail.mozilla.org&#x2F;pipermail&#x2F;tb-planning&#x2F;2015-September&#x2F;004066.html" rel="nofollow">https:&#x2F;&#x2F;mail.mozilla.org&#x2F;pipermail&#x2F;tb-planning&#x2F;2015-Septembe...</a>