Glossary

Every concept in this sheet, plus the smaller terms it uses. Select a concept to open its card.

Accessible forms: Every input has a visible label tied to it with for and id (or by nesting), the right type and autocomplete, and errors linked with aria-describedby so screen readers announce them.
Alt text: The alt attribute describes what an image communicates.
ARIA: Attributes such as role, aria-label, aria-expanded and aria-live that add or override accessibility information where HTML alone cannot express it.

Colour contrast: WCAG AA asks for a contrast ratio of at least 4.5:1 for normal text and 3:1 for large text and interface parts such as input borders and focus rings.
CORS: Browsers block JavaScript from reading responses from another origin (scheme, host and port) unless that server sends headers such as Access-Control-Allow-Origin allowing it.
Critical rendering path: The browser builds the DOM from HTML and the CSSOM from CSS, combines them into a render tree, calculates layout (sizes and positions), paints pixels and composites layers.

DNS: The internet's phone book: it turns a name like example.com into an IP address through a chain of resolvers, and caches the answer for the record's TTL (time to live).

From URL to page: The browser resolves the domain to an IP address with DNS, opens a connection (TCP or QUIC), negotiates TLS, sends an HTTP request, then parses the HTML response and fetches the CSS, JavaScript and images it references.

Headings and landmarks: One h1 per page and headings in order without skipping levels, plus landmark elements (header, nav, main, footer) that outline the page.
HTTP methods: The verb of a request: GET reads, POST creates or triggers an action, PUT replaces, PATCH partially updates, DELETE removes.
HTTP status codes: Three digit results of a request: 2xx success (200 OK, 201 Created, 204 No Content), 3xx redirect (301, 304 Not Modified), 4xx client error (400, 401, 403, 404, 429) and 5xx server error (500, 502, 503).
HTTPS and TLS: HTTP over TLS, which encrypts traffic and proves with a certificate that you are talking to the real domain.

Keyboard navigation and focus: Everything interactive must be reachable with Tab in a logical order and show a visible focus style.

Semantic HTML: Using elements for their meaning: header, nav, main, article, button, a, h1 to h6, ul, table, instead of generic div and span for everything.