After creating a website, one of the first things done is to run W3C validators to see if we coded HTML-CSS according to web standards.

But, although it is easy to perform this task for a website with few pages, this is pretty difficult and time-consuming for any larger sites.

Simply Testable is a free-to-use web application that automates the HTML validation part by validating all the pages of a website.

The test process is displayed live (how many pages are checked , how many are queued, etc.) and a report is generated once the test is completed.

Reports include any errors for each URL with explanations so that we can easily go through them and fix each.

P.S. CSS validation and JavaScript static analysis is a planned feature.

CSSHórus is a CSS framework for quickly creating responsive and mobile websites that works cross-browser.

The framework has a 16 column grid and base styles for the reset, typography, tables, lists, forms and buttons.

It is available as both CSS + LESS and includes 960px + 1200px grids in PSD format for designing easier.

There is support for all major browsers starting from IE7+ and platforms lik iOS, Symbian, Windows Phone and more.

We probably all know Twitter Bootstrap, the exciting, feature-rich framework (by Twitter) for easily/quickly building websites.

BootMetro is another one, which is built on top of Twitter Bootstrap, for creating Metro/Windows 8-styled websites.

The framework is inspired form the Metro UI CSS and provides a pretty solid base for the fans of this (possibly to become popular) UI style.

It includes all the features, components + plugins of Bootstrap and extras like the tiled pages, application bar and more.

CSS FilterLab is a free-to-use online application, created by Adobe, for discovering CSS Filters & CSS Custom Filters.

It allows us to apply any number of filters from a list, customize their values, see the output instantly and get the CSS rules generated.

There is also an "animate" feature which makes it possible to create keyframe based animations using the filters and save them for future use.

The application is also open sourced and available through GitHub.

If you need a base when starting a new project and don't want it to be complicated, Kube Framework can be a good choice.

The framework is a single CSS file that brings responsive layouts with no headache and offers more.

Kube includes styles for forms, grids, buttons, tables, typography and few other stuff like links or images.

For developers looking for maximum flexibility and customization, LESS files are also provided.

Base is a really simple CSS framework for building responsive websites that work well in all devices.

It comes with custom styles for headings, tables, blockquotes forms, lists and more.

There is a 960px (max), 12-column grid and it is created with screen readers in mind.

The CSS rules are well-commented and supports all major browsers (IE7+).

After so many years, Windows OS is having a huge overhaul with its Metro UI and it seems like their new style may spread to web apps as well.

Metro UI CSS is a set of styles for creating such interfaces. It is a self-contained solution but can also be used side-by-side with any other frameworks.

There are styles for the popular tiles, images, notices, forms, buttons and typography.

The framework is also a "work in progress" and is mentioned to be enriched with more layout options + features.

When writing CSS code, adding CSS properties in a specific order for each rule helps reading it easier so much.

For anyone who already does that or not, CSScomb, a free service sorts CSS properties in any order you want (and, their default order is pretty good too -from the most to least important-).

It works with both single + multi-line CSS code and besides the online version, there are add-ons provided for the most popular IDEs.

If you are a web designer/developer who first creates wireframes for a web project, Wirefy will help you a lot.

It is a CSS-JS framework for creating responsive and cross-browser wireframe layouts very quickly.

Wirefy is style-agnostic so that clients won't get hung up on the look or colors but just the layout and -maybe- functionality.

The framework includes styles for all major web design elements like typography, grid, tables, images, forms, buttons, pagination,slideshow and more.

ResponsiveAeon is a HTML5/CSS3-powered framework for creating responsive layouts very quickly.

It has a grid system that is based on 12 columns with 1104px width in total and offers a very easy to understand usage with only 3 basic classes.

The framework makes use of @mediaqueries and can work with any device in any size.

Besides the grid system, there are also styles for typography, lists, tables, buttons and forms.

The download package includes several JavaScript files for IE compatibility, modal boxes and also PSD/AI/PNG files of the grid.