Mobile and tablet devices Guidelines 1.3.4, 1.4.10
Websites must only scroll in one axis at once on a small screen. They must work in both landscape or portrait, as users may not be able to rotate their device (e.g. if attached to a wheelchair).

Automatic completion Guideline 1.3.5
Modern browsers can fill in common information like your name and email address for you. You must now support this capability where applicable.

Better legibility Guidelines 1.4.11, 1.4.12
The old standard required text to have enough colour contrast to be easy for everyone to read. This has now been expanded to cover things like the spacing between letters and lines, plus the contrast of buttons, form controls, and more.

Alternative modalities Guidelines 2.5.1, 2.5.2, 2.5.4
Websites must provide an alternative to performing gestures (such as swiping), using multiple fingers at once (e.g. pinch-to-zoom), moving their device (e.g. tilting their phone), or drag-and-drop. Accessible alternatives must be provided, e.g. buttons to scroll or zoom.

Large enough to touch Guideline 2.5.5
Areas of your website that can be clicked or tapped must be sufficiently large, especially on mobile devices. This aids users with visual and motor impairments.

Accessible statuses Guideline 4.1.3
Specific technologies to alert a user to content they may not be able to see, such as confirmation that a form is submitted or an error message.