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As an #NHSComms person, I talk about accessibility a lot. The most common mistakes that I come across that are easy to fix? Here's a list 
Fixable problem 1: Alternative (alt) text for images. These are used by screen readers or load when images don't. Describe the content and function of the image. If it's purely decorative use alt=" ". Better yet: don't use decorative images. More info: https://bit.ly/2N5c2Lg
Fixable problem 2: Poor hyperlink copy. When you link to something, your link needs to describe where the person is going to go when they click it. "Click here" doesn't cut it. Great example here: https://bit.ly/3rWw6y4 via @w3c_wai
Fixable problem 3: Over reliance on Powerpoint/slide decks. Visual communications like this are difficult to navigate if you use a screen reader. Info about how to make your presentations accessible: https://bit.ly/3bdClXs
Fixable problem 4 (and my biggest current bugbear): Moving text in Instagram stories. The typing as you watch aesthetic is cute but so hard for people with visual and learning issues to engage with. Static text should be your default. Please.
There's a wealth of information about accessibility out there. Here's Stephanie Hill from the Government Communications Service (GCS) discussing her most common accessibility queries: https://gcs.civilservice.gov.uk/blog/are-these-the-most-asked-questions-about-accessibility/ All hail, @UKgovcomms 
It should go without saying but I'll say it anyway: accessibility on the web should be something that everyone thinks about. Think about the people that read and consumer your content in all of their glorious forms.
