What JWT, Base64, UUID, HEX color, WCAG contrast, translit and amount in words mean — in plain language, with a link to the matching tool.
What is a JWT (JSON Web Token): header.payload.signature structure, claims, and how to decode a token online in the browser.
What is Base64: how binary-to-text encoding works, the URL-safe variant, and online encode/decode.
What is a UUID (GUID): a 128-bit identifier, versions v1–v7, and why APIs and databases use it.
What is UUID v7: Unix time at the start of the ID, database sorting, and how it differs from random UUID v4.
What is JSON: a text format for objects and arrays used in APIs and configs, plus an online validator.
What is a Unix timestamp (epoch): seconds or milliseconds since 1970-01-01 UTC and how to convert to a date online.
What is haversine: great-circle distance between two lat/lon points and an online distance calculator.
What is password entropy in bits: why it matters for security and how to score a password online.
What is a MIME type (Content-Type): labels like image/png and application/json and why the web needs them.
What is an ASN on the internet: an autonomous system number for routing and how it relates to IP and ISP.
What is VAT: how to add or extract tax from a price and an online VAT calculator by rate.
What is devicePixelRatio (DPR): physical pixels vs CSS pixels on Retina and HiDPI screens.
What reaction time is: milliseconds from a visual cue to a click, typical ranges, and an online test.
What a photo watermark is: text or logo over an image for copyright and drafts, plus an online tool.
What photo compression is: shrink JPG, WebP and AVIF file size, lossy vs lossless, and how to compress an image online.
What aspect ratio means for photos: square, 4:5 Instagram, 16:9 video, and how to crop online.
What srcset is: how browsers pick a source from several image URLs, and how to extract those URLs from a page.
What EXIF is: camera settings and GPS in a JPEG, why re-encoding drops it, and how to compress a photo without uploading.
What DPI means for a passport photo: dots per inch, millimetres to pixels, and 300 vs 600 DPI.
What a QR code is: modules, error correction, Wi‑Fi and URL payloads, and how to generate one online.
What a HEX color is: short form, HEX8 with alpha, how it maps to RGB, and an online converter.
Text contrast in WCAG 2: relative luminance, the 4.5:1 and 7:1 thresholds, and an online check.
Russian translit to Latin: how it differs from translation, GOST and passport tables, online tool.
Amount in words: why contracts repeat figures as text, cents and currency, online converter.
Keyword density: how the share of hits is counted, why a phrase beats a single token, online counter.
