Validate your HTML and CSS file using W3C markup validator or W3C CSS validator.

The package validate on save by default (you can disable this behavior in the settings). You can also use the comma,d w3c-validation:validate.

Note: the validation process is made by requesting the code to the online W3C markup validator or W3C CSS validator. This can take some time.

validateOnSave: Make a validation each time you save an HTML file.

validateOnChange: Make a validation each time you change an HTML file.

hideOnNoErrors: Hide the panel if there was no errors.

useFoldModeAsDefault: Fold the results panel by default.

cssProfile: Profile to use for CSS file validation (default: css3). For CSS files only

cssMedia: Media to use for CSS file validation (default: all). For CSS files only

cssReportType: CSS Report severity (default: normal). For CSS files only

With the success of Atom, it's really difficult to choose keybindings that will not enter in conflict whit anyone else's packages, so I have removed the default keystrokes and let the keymap empty to let you set your own keybindings.