At least posteo.de web email client renders colors with the first value given in a CSS style definition, which yields unreadable buttons in notifications:
instead of
Currently, the according style is
font-weight: normal;; text-decoration: none; font-weight: bold; color: #006699;; background-color: #2F70AC; color: #FFFFFF; border-top: 4px solid #2F70AC; border-right: 6px solid #2F70AC; border-bottom: 4px solid #2F70AC; border-left: 6px solid #2F70AC; display: inline-block; font-weight: bold;
If changed to include only the latest definitions
font-weight: normal; text-decoration: none; font-weight: bold; background-color: #2F70AC; color: #FFFFFF; border-top: 4px solid #2F70AC; border-right: 6px solid #2F70AC; border-bottom: 4px solid #2F70AC; border-left: 6px solid #2F70AC; display: inline-block; font-weight: bold;
one gets the desired behavior.
Since there is no benefit by sending later overwritten styles, rendering problems with buggy email viewers could be resolved by sending only the final CSS definitions.