Email supports multiple MIME types, so if someone sends you an HTML email, they should also be converting that HTML into a text representation as well so you can choose (or your email client can choose) which version you want to read.

Converting from HTML to text isn't that difficult, there are a number of open source libraries that'll do it. The harder problem is if someone sends you an email with a lot of images or where the layout of content is important, which is why people typically use HTML, it's much harder to map those aspects to plain text.