If you just want text and images, you don't need CSS. Just the <img> tag - and even there you'll run into differences if you try and use the alt attribute. And

If you just want text and images, you don't need CSS. Just the <img> tag - and even there you'll run into differences if you try and use the alt attribute. And even that isn't trivial. For example, say you include an basic black on white icon, what happens if the user is using dark mode? If the image is transparent, it becomes entirely invisible. If it's non-transparent then it's ugly (white box). If you want to make it change depending on dark or light mode, now you have to use responsive CSS which works entirely differently in different mail clients. What happens if the image is bigger than their viewport? Hint: it is different in different clients. And speaking of mail clients: one of the most popular mail clients is Outlook, which on Windows uses the Microsoft Word(!) HTML renderer and on Mac it uses WebKit. Gmail on web has its own HTML parser that outputs different HTML to what you wrote (and mangles your CSS class names). Apple Mail is probably the easiest to deal with. It's not worth learning this stuff if you don't have to. It will not benefit your life in any way. The common subset you want is text/plain.