Browsers use the DOCTYPE declaration to choose whether to show the document using a mode that is more compatible with Web standards or with old browser bugs. Using a correct and modern DOCTYPE declaration at the start of your HTML will improve browser standards compliance.

HTML elements can have an id and/or class attribute. The id attribute assigns a name to the element it is applied to, and for valid markup, there can be only one element with that name. The class attribute assigns a class name to the element, and that name can be used on many elements within the page. CSS allows you to apply styles to particular id and/or class names.