MDN HTML
This page contains details on what HTML is, tutorials on how to use HTML, and references for HTML elements. This page can be used to reference elements available when writing the structure of your website.
MDN CSS
This page contains details on what CSS is, tutorials on how to use it, and references for what each property does. This page can be used to reference each property and value available with CSS.
MDN DOM
This page contains details on what the Document Object Model is, and references to each DOM interface. This page can be used to reference how to write scripts that utilize the DOM.
MDN JavaScript
This page contains details on what JavaScript is, as well as tutorials on how to use it. It can be used to review correct JavaScript syntax and troubleshoot errors.
Summary
A trusted resource is important when looking for programming help, because it will provide up to date information in a consistent format. Additionally, there are often many solutions to a coding problem, but not all solutions are efficient.