Tools
Open a card for a full guide and a browser-only tool — no uploads, no server round trips. Curated external picks sit below.
In-browser tools
3 live · runs in your tabJSON format & minify
Validate, pretty-print, or compress JSON for configs and APIs.
Unix time converter
Parse ISO strings or Unix seconds/ms into local time and UTC.
UUID generator
Random UUID v4 values for fixtures, IDs, and placeholders.
External picks
9 curated · opens in a new tabBundlephobia
Check npm package size and dependency cost before you install.
Can I use
Browser support tables for HTML, CSS, and JavaScript features.
Chrome DevTools
Performance panel, coverage, and memory tooling in the browser.
GTmetrix
Performance scores, waterfalls, and historical monitoring for a URL.
PageSpeed Insights
Lab + field data from Lighthouse for any public URL.
Squoosh
Compress and convert images in the browser before you ship them.
W3C CSS Validator
Check CSS for errors and warnings.
W3C Markup Validator
Validate HTML documents for syntax and parse errors.
WebPageTest
Filmstrip, waterfalls, and multi-location runs.
What “in-browser” means
Every built-in tool runs in your tab — no payload leaves your browser, no analytics tracks what you paste. Hashes use the Web Crypto API, JSON parses with the engine’s native parser, and tokens are decoded with atob plus a TextDecoder. We mention what each tool does not verify on its own page.