When your user base consists of millions of people from pretty much every country, there are many things to consider while conducting user tests.
HTML Signatures for Apple Mail
There are plenty of tutorials online to create an HTML signature in Apple Mail with older versions of OS X, but the process has changed ever so slightly for the new macOS X Sierra (10.12.x). Matt Coneybeare has a great blog post and shows how to do it.
Beginner’s Guide to HTML & CSS
A Beginner’s Guide to HTML & CSS is a simple and comprehensive guide dedicated to helping beginners learn HTML and CSS. Outlining the fundamentals, this guide works through all common elements of front-end design and development.