Images

alt tag

Same old same old, but I think even the best developers miss this step. It's important for screen readers and people that use them to have a descriptive text to get the same experience as other users. If it's a decorative image then have it empty,alt="" or include them in the CSS instead.

aria-labelledby

If the image contains a lot of content that is important for the user, you can use the attribute aria-labelledby that refers to the content. That forces the screen readers to use that paragraph as the alt text/label for that image. This is useful if you want to have the same text for multiple images on the same page.

longdesc

There is an attribute for the image that is called longdesc, but after much research I discovered that it's not readable by all screen readers. So use aria-labelledby insted.