Viewport
beginnerDefinition
The slice of a webpage you can actually see in the browser window. On a phone, the viewport is small; on a wide monitor, it's large. Designers think carefully about the viewport because the same page has to look right at every possible size.
In the wild
Open a website on your phone, then on a desktop. The page knows your phone has a small viewport and gives you a single column with big tap targets. The same page on the desktop's larger viewport spreads out into multiple columns and uses smaller, mouse-sized buttons.
More from Frontend & Responsive Design
Accessibility (a11y)
Building websites and apps so that everyone can use them: including people who can't see well, can't use a mouse, are color-blind, or rely on a screen reader to read pages aloud. Accessibility is partly thoughtful design and partly small technical details done correctly.
ARIA (Accessible Rich Internet Applications)
A set of small labels you can add to webpage elements to tell screen readers and other assistive tools what each piece of the page is and what it's doing. ARIA is useful when the standard tags can't fully explain a custom widget on their own.
Box Model
The mental picture of how every element on a webpage is laid out. Each element is a rectangle made of four nested layers: the actual content in the middle, padding around it (the inner cushion), then a border, then margin (the outer breathing room before the next element starts).
Breakpoint
A particular screen width at which a webpage changes its layout to suit the new size. Below the breakpoint the page might show a single column for phones; above it, the same content might rearrange into a multi-column layout for tablets or desktops.
CSS Grid
A way of arranging things on a webpage into neat rows and columns at the same time: like a spreadsheet you can place items into. It's the right tool for page layouts, photo galleries, and anything that needs to line up in a tidy grid.
CSS Specificity
The way a webpage decides which style wins when several styling rules try to apply to the same element. The more precise a rule is, naming a particular item by ID rather than a whole group, the higher its priority. It's why a designer's careful tweak sometimes loses to an older, more specific rule they forgot existed.