CSS所能做的就是改变网页的布局、排版和调整字间距等，但编写 CSS 并不是一项容易的任务，当你接触新的 CSS3 属性及其各自的浏览器前缀的时候，你会发现很伤脑经。值得庆幸的是一些优秀的开发人员提供了很多在线的 Web 应用程序来帮助我们。
1. Skyline
Skyline helps you create a design system for your website. It encourages a well thought-out CSS architecture, and enables you to develop a reusable and maintainable codebase. The starter kit provides a solid CSS architecture that acts as a scaffolding to support your unique design system. It promotes object-oriented CSS, written in SCSS using BEM notation.
2. CSSess
CSSess is a JS tool that helps find extra CSS rules on your site. It is designed to be used as a bookmarklet. Each URL is loaded into a hidden iframe. Once the iframe has finished loading all of the stylesheets and inline styles are checked against the DOM to find unused styles.
3. Buddy
Buddy is a build tool for js/css/html projects. It compiles source code from higher order js/css/html languages (CoffeeScript, es6, JSX, Handlebars, Dust, Nunjucks, Stylus, Less, Jade, Twig), automatically wraps js files in module definitions, statically resolves js/css/html dependencies, and concatenates (and optionally compresses) all souces into bundles for more efficient delivery to the browser.
4. CSS Burrito
CSS Burrito is an organizational Sass template that includes four main elements: application.scss, libs, six core components, and modules. It’s goal is to keep your code organized so that projects will scale better.
5. SpaceBase
SpaceBase is a Sass-based responsive CSS framework. Many CSS frameworks are meant to be included and left alone. But spaceBase is a boilerplate layer that can be built upon and tailored for your needs. It combines best practices for today’s responsive web with the core components we use on every project. Consider it the launch pad for your adventures into cyberspace.
6. Contrast Ratio
A tool to calculate the contrast ratio between any two valid CSS colors
7. Penthouse
Penthouse is a critical path CSS generator for speeding up web page rendering. It looks at your full CSS in relation to the page and provides the CSS needed to render the above-the-fold content of your page, saving time and server resources.
8. Iconizr
A PHP command line tool for converting SVG images to a set of CSS icons (SVG & PNG, single icons and / or CSS sprites) with support for image optimization and Sass output. Created by Joschi Kuphal (@jkphl), licensed under the terms of the MIT license.
9. deSVG
deSVG makes it simple to remove inline SVG bloat from your HTML. It allows you to style your SVGs with CSS, and keeps your SVGs accessible even without JavaScript.
10. Strip CSS Comments
Strip CSS Comments makes it easy to do just that: remove comments from your CSS files, reducing file sizes. It’s also available as a gulp/grunt/broccoli plugin.
11. SkyBlue
SkyBlue is a CSS framework that offers a grid, typography, tables, forms, buttons, and icons.
12. CSS Critic
A lightweight tool for regression testing of Cascading Style Sheets, CSS Critic checks your current layout constantly against a reference image you have provided in the past. If your layout breaks (or simply changes – CSS Critic can’t tell) your tests fail.