Universal Design is a concept—coming originally out of architectural design, coined as a term by Ronald Mace—that has been widely applied to accessibility in design contexts.
Universal Design, broadly speaking, seeks to meet the access needs of as many of a design's potential users as possible, without requiring users to make additional adaptations. Universal Design can benefit abled users, as well as disabled users: a commonly cited example is the usefulness of curb cuts, invented by Selwyn Goldsmith, for both wheelchair users and abled people with strollers. However, as various disability studies scholars have pointed out—for more on this, see Jay Dolmage's chapter on Universal Design—framing Universal Design this way can water down the emphasis on access for disabled users.
Thinking about your digital humanities project through a lens of Universal Design (especially if you remember to center disabled users in your Universal Design) can help you build more accessible, and often more interesting, projects.
Anything that flashes more than three times per second, including animations and videos, poses a seizure risk, and is therefore physically dangerous to some users. PEAT is a free Windows tool that can help you identify this risk, especially if you have any quick or high-contrast videos or animations (lightning, quick transitions, etc.) in your web or desktop projects.
These are not meant to be a complete list of considerations for your projects' accessibility, but rather to offer a set of key examples of common considerations to make for digital humanities projects.