When you add alt text to an image on Twitter, users with screen readers will have that alt text read to them.
To add alt text on your Instagram photos, use the "Advanced Settings" link after you choose a filter when creating a post.
For reference, Google Photos' video editor currently only lets users select one default font for their text overlay needs, paired with a slightly more generous color wheel consisting of seven options.
Some results have been hidden because they may be inaccessible to you
Show inaccessible results
Feedback