Part Three: The details · Before anything else
It is quickest to design for good eyesight, steady hands and a mouse. A great many people have something else, permanently or just today.
The person a team imagines while designing is usually someone like themselves: sharp eyes, two free hands, a large screen, a quiet room. Design for that person and the work goes fast. Text can be small and pale. Targets can be tiny. Everything can depend on dragging, hovering, or telling red from green.
Someone who is blind uses a screen reader, which reads out what the code says, and the code says "button, button, image." Someone with a tremor cannot land on a small icon. Someone older has enlarged the text, and the layout has collapsed. These people are locked out of whatever the product is the door to: a job application, a bank account, a doctor's appointment. When the product is a tool an employer requires, they are locked out of the job itself.
Teams file this under "edge cases" and schedule it for later. Microsoft's inclusive design work offers a better way to see it. A limitation can be permanent, temporary or situational. One person has one arm. Another has a broken wrist this month. A third is holding a baby. All three need the product to work with one hand. Sunlight on a phone screen is low vision for an afternoon. A noisy train is deafness for a commute. Counted that way, nearly everyone is included at some point, and what helps one group helps the rest.
It is far cheaper at the start than as a repair, because most of it lies in ordinary decisions: real buttons, real headings, labels on fields, and body text with a contrast of at least 4.5 to 1 against its background.
Therefore:
From the first screen, make everything work with a keyboard alone, with a screen reader, with enlarged text and without relying on color. Treat a failure in any of these as a defect.
In practice. Put the mouse in a drawer and do your product's main task with the keyboard. Then zoom the browser to 200 percent and do it again. Note every place you get stuck.
Goes with. A phone held in one hand is a constraint too: WORKS WHERE THEY ARE (47). On screen it depends on OBVIOUSLY CLICKABLE (72). To find out where you stand, WATCH SOMEONE USE IT (59), including someone on a screen reader.
Sources: Microsoft, Inclusive Design toolkit (2016). The contrast figure is from the Web Content Accessibility Guidelines, level AA.