HTML is a standard language, which should mean that all web browsers interpret it in exactly the same way. The standard is decided upon by a large body of volunteers and experts, who oversee its progress and guide its development. This body is managed by the World Wide Web Consortium (the W3C, http://www.w3.org/), and currently recommends the use of HTML 5.0 latest standard).

One of the reasons that the web is so widely used is that the standardisation of HTML has allowed many developers to produce browser software, for many types of computer. The original definition of HTML was written in a language called SGML (Standard Generalised Markup Language), designed to allow computers to read language definitions easily. This means that HTML is defined very precisely, which is why you need to be equally precise when writing your web pages. If a web page is written in such a way that it conforms to the SGML definition, then it should be readable by all browsers conforming to that definition. Checking that your document conforms to the definition of, say, the HTML 5.0 standard, is called validation, of which more below!