I use Thunderbird on my late `09 Mac Mini. Last week I was testing an html email that uses Bootstrap css for the design layout and it looked great in Thunderbir

I use Thunderbird on my late `09 Mac Mini. Last week I was testing an html email that uses Bootstrap css for the design layout and it looked great in Thunderbird, Gmail strips any tags for links to css files, and so does Roundcube, so it looked like crap in those. So then I tried embedding the css in the email and same thing. Thunderbird rendered it perfectly and the others ignored and/or removed it. I'm at a loss as to why CSS in an email is ignored or stripped out by Gmail. Sure makes it ugly though.