Discuss the differences between an HTML specification and a browser’s implementation thereof.

HTML specifications such as HTML5 define a set of rules that a document must adhere to in order to be “valid” according to that specification. In addition, a specification provides instructions on how a browser must interpret and render such a document.

A browser is said to “support” a specification if it handles valid documents according to the rules of the specification. As of yet, no browser supports all aspects of the HTML5 specification (although all of the major browser support most of it), and as a result, it is necessary for the developer to confirm whether the aspect they are making use of will be supported by all of the browsers on which they hope to display their content. This is why cross-browser support continues to be a headache for developers, despite the improved specificiations.

In addition, while HTML5 defines some rules to follow for an invalid HTML5 document (i.e., one that contains syntactical errors), invalid documents may contain anything, and it is impossible for the specification to handle all possibilities comprehensively. Thus, many decisions about how to handle malformed documents are left up to the browser.

Briefly describe the correct usage of the following HTML5 semantic elements: <header>, <article>, <section>, <footer>.

The <header> element is used to contain introductory and navigational information about a section of the page. This can include the section heading, the author’s name, time and date of publication, table of contents, or other navigational information.

The <article> element is meant to house a self-contained composition that can logically be independently recreated outside of the page without losing it’s meaining. Individual blog posts or news stories are good examples.

The <section> element is a flexible container for holding content that shares a common informational theme or purpose.

The <footer> element is used to hold information that should appear at the end of a section of content and contain additional information about the section. Author’s name, copyright information, and related links are typical examples of such content.