5. Don’t use CSS at all.
If you’re unsure or cautious about sending HTML emails with CSS, there is a simple solution: plain-text emails.

Most of the big email providers (including Gmail and Outlook) already allow users to see a plain-text version of your email if you are using CSS code that is not recognized. But you could take it one step further and just send all of your emails using basic HTML or plain text.

While they do not look flashy and definitely won’t win any design awards, this might be the best way to get your message across. Take a look at this example from Nintendo:
https://www.campaignmonitor.com/css/?__hstc=53776247.f6daf62fa5fa01383b025fbeed693c7e.1659146465716.1659146465716.1659146465716.1&__hssc=53776247.1.1659146465716&__hsfp=3897503624