Aspect ratio of a width and height (simplified, nearest common ratio) or the missing side for a ratio, with CSS values.

CSS selector specificity (ids, classes, types) per Selectors Level 4, including :is(), :not(), :where() and :has(); compare two.

Convert a CSS length between px, rem, em, pt, pc, in, cm, mm, Q, vw, vh, vmin, vmax and % with your root and viewport sizes.

Split a length into major and minor parts by the golden ratio (or silver, root-2, custom) with matching rectangle sizes.

Screen pixel density (PPI) from resolution and diagonal, physical size, CSS viewport at a device pixel ratio, and image widths.

HTTP status code meaning, typical cause, client action and cacheability, from the IANA registry and RFC 9110.

Parse a Cache-Control value, explain each directive, compute freshness and flag conflicts or leaks.

All registered HTTP status codes in a class (1xx to 5xx) with names and one-line meanings.

Modular type scale: sizes in px and rem for steps below and above a base size, by named ratio (major third, golden) or number.

Convert a coordinate pair between decimal degrees, degrees-minutes-seconds and degrees-decimal-minutes.

Unicode character lookup by code point or character: name, block, category, UTF-8/UTF-16 bytes and HTML/JS/CSS escapes.

IANA service name and common use for a TCP/UDP port (about 130 well-known ports), plus the port range class.

Snap line-heights for font sizes to a baseline grid: baseline unit, lines per element, unitless line-height and margins.

Parse a Content-Security-Policy header value, explain each directive and flag risky settings like unsafe-inline.

Convert a timestamp (RFC 3339, ISO 8601, HTTP-date or epoch seconds) to RFC 3339, HTTP-date, RFC 5322, basic ISO and epoch.

Bounding box (min/max latitude and longitude) enclosing a radius in meters around a point, with pole and 180 degree handling.

Media type (MIME) for a file extension, or extensions for a media type, with IANA registration status.

Whether an HTTP method is safe and idempotent (IANA registry), what it is for, and request-body semantics.

Explain an HTTP header: request or response, purpose, security notes, and its IANA registration status.

DNS record type lookup by name (MX) or number (15): IANA meaning, RFC reference and typical use.