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
11 live · runs in your tabBase64 encode & decode
Turn text into Base64 and back — UTF-8 safe, in your tab.
HTML entity encode & decode
Escape text for HTML or decode common entities — client-side only.
JSON format & minify
Validate, pretty-print, or compress JSON for configs and APIs.
JWT decoder
Inspect header and payload JSON — signature is not verified here.
Meta description length
Count characters and aim for a snippet-friendly range.
SHA-256 hash
Fingerprint arbitrary text with SHA-256 — no server round trip.
Unix time converter
Parse ISO strings or Unix seconds/ms into local time and UTC.
URL encode & decode
Percent-encode query values or decode escaped strings.
URL slug generator
Turn titles into clean path segments for blogs and static routes.
UUID generator
Random UUID v4 values for fixtures, IDs, and placeholders.
WCAG contrast checker
Test color pairs against WCAG contrast ratios.
External picks
26 curated · opens in a new tab@cometweb/carbon-badge
Lightweight website carbon footprint badge for your pages — npm package for teams and static sites.
Accessibility Insights
Fast checks and guided assessments from Microsoft.
axe DevTools
Browser extension and APIs used in many CI pipelines.
Bing URL Inspection
See how Bing crawls and indexes a URL.
Bundlephobia
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.
CSP Evaluator
Paste a Content-Security-Policy and spot risky directives.
Ecograder
Sustainability score with actionable recommendations.
Google Search Console
Coverage, indexing issues, and Core Web Vitals for properties you verify.
Green Web Foundation
Directory and APIs for green hosting verification.
GTmetrix
Performance scores, waterfalls, and historical monitoring for a URL.
Mozilla Observatory
HTTP Observatory scan with a simple letter grade.
Pa11y
CLI and dashboard-friendly automated accessibility testing.
PageSpeed Insights
Lab + field data from Lighthouse for any public URL.
Qualys SSL Labs
Deep TLS configuration and certificate chain analysis.
Rich Results Test
Validate structured data Google can use for rich results.
Schema Markup Validator
Schema.org validation for JSON-LD, Microdata, and RDFa.
Security Headers
Quick report on HSTS, CSP, and related response headers.
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.
WAVE
Visual feedback on structure, contrast, and ARIA on live pages.
WebAIM Contrast Checker
Check foreground/background contrast ratios against WCAG levels.
WebPageTest
Filmstrip, waterfalls, and multi-location runs.
Website Carbon Calculator
Rough CO₂ estimate per page view and hosting context.
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.