One page, many readers, none of them identical to you

You build on one screen with one set of abilities, and then the page goes out to everyone else.

The hardest thing about building for the web is that you cannot experience your own work the way most people will. You see it at your window size, with your fonts loaded, your images arrived, your connection fast, your eyesight whatever it happens to be, and your hand on a device you are comfortable with. Somebody else is reading the same page at four times the text size, or with the pictures switched off to save data, or by listening, or with a keyboard alone because a mouse is not usable for them, or on a screen the size of a matchbox, in sunlight.

You cannot anticipate all of those situations individually, and you should not try. What you can do is stop making the page depend on any one of them. A correct description survives all of them because each reader interprets it in the way that suits their circumstances. Text that is genuinely text can be enlarged. An outline that is genuinely an outline can be jumped through. A control that is genuinely a control can be reached by keyboard because that behaviour comes free with the element. The variation is handled by the readers, not by you, provided you gave them something truthful to work with.

This is why the discipline pays off in situations that have nothing to do with anybody's abilities. Structure written honestly also survives a redesign, a change of layout, a move to a different presentation entirely, and the arrival of some new kind of device nobody has thought of yet. You are not writing for the screen in front of you. You are writing a description durable enough that a machine you will never see can render it sensibly. That is a genuinely different job from making something look right today, and it is the more valuable one.