Awesome Bookmarklets 
Table of contents
Accessibility & Validation
a11y.css
This CSS file intends to warn developers about possible risks and mistakes that exist in HTML code. It can also be used to roughly evaluate a site's quality by simply including it as an external stylesheet.
HTML_CodeSniffer
HTML_CodeSniffer is a client-side JavaScript that checks an HTML document or source code, and detects violations of a defined coding standard.
Readability Bookmarklet
This tool will show you how readable selected text is on a page. The readability guidelines are based on WCAG 2 Level AAA Conformance. Note, there is no readability requirement for WCAG 2, Level AA Conformance. The results are based on six tests.
REVENGE.CSS
The premise of revenge.css is simple: A CSS bookmarklet that uses selectors to find bad markup, displaying ugly pink error messages in comic sans serif wherever you write bad HTML.
Web Evaluation Tools Bookmarklet
This set of tools allows you to check for several aspects of Web accessibility that other tools do not. It is not designed to replace any other tools, but rather to supplement them.
Debug
45-75
Check whether lines are between 45 and 75 characters, for optimal readability; this especially helps with font sizing for responsive web design.
CSS Debugger
CSS styles to highlight problematic and malformed HTML
CSS Layout Debugger
A tweet-sized debugger for visualizing your CSS layouts. Outlines every DOM element on your page a random (valid) CSS hex color.
Fonts
FFFFALLBACK
Web fonts are here, sparking an exciting new era in web design. FFFFALLBACK makes it easy to find the perfect fallback fonts, so that your designs degrade gracefully.
FontFriend
FontΒFriend is a bookΒmarklet for typoΒgraphΒiΒcally obsessed web designΒers. It enables rapid checkΒing of fonts and font styles directly in the browser withΒout editΒing code and refreshΒing pages, makΒing it the ideal comΒpanΒion for creΒatΒing CSS font stacks.
Fount
Identify webfonts on any site.
Flippant
View any site from all angles.
MIN
Basic looks at type and layout.
WebFontPicker
A bookmarklet that helps preview Google Web Fonts live on any website.
WhatFont
WhatFont is a JavaScript script to detect what font in the stack is being used on any elements of a web page.
Performance
CriticalCSS
CriticalCSS Bookmarklet and Devtool Snippet.js
CSS Stress Test
This project is a bookmarklet for stress testing the CSS on any given webpage. It indexes all the elements and their classes, and then--class by class--it removes one, and times how long it takes to scroll the page. Selectors that save a considerable amount of time when removed indicate problem areas.
pagespeedThis
Analyses the pagespeed of the current page
perfmap
A bookmarklet to create a front-end performance heatmap of resources loaded in the browser using the Resource Timing API. A browser with support for the Resource Timing API is required.
Performance-Bookmarklet
Bookmarklet to analyze the current page through the Resource Timing API, Navigation Timing API and User-Timing - requests by type, domain, load times, marks and more - sort of a light live WebPageTest.
stats.js
This class provides a simple info box that will help you monitor your code performance. FPS Frames rendered in the last second. The higher the number the better. MS Milliseconds needed to render a frame. The lower the number the better.
timing.js
Timing.js is a small set of helpers for working with the Navigation Timing API to identify where your application is spending its time. Useful as a standalone script, DevTools Snippet or bookmarklet.
Yslow
YSlow analyzes web pages and why they're slow based on Yahoo!'s rules for high-performance websites
Search Engine Optimization
OuiSEO
Displays SEO and social metadata information.
SEO Bookmarklet
A One-Stop SEO Bookmarklet to Quickly Review On-Site SEO
Games
Astroids
Play Astroids on a website and destroy it.
