HTML means Hyper Text Markup Language and is the main language used for web pages. HTML data contain normal text merged with marks like <h1> (means "start of header first level") and </h1> (means "end of that header"). Marks can be used to describe size and color of fonts or to insert images and so on or to define a hyperlink, that is a link to another HTML page or to another section of the current page, which allows to continue reading at another place.
It is possible to describe a web page totally by means of HTML, but it is not appropriate, because a change of style requires to find each occurence of all these marks in order to change them. The better solution is to separate the content and the representation of the web page. Therefore, HTML contains only tags like <h1> ... </h1> and similar start-and-stop-of-anything marks.
CSS ist the language used for the detailed interpretation of the HTML-tags. We know, <h1> ... </h1> has to be the first level header, but the description of font, size, color and many other options to design that header, that ist done via CSS and valid for any header used in the whole web page.
At the moment, 3 browsers dominate the marketplace: Internet Explorer, Firefox and Chrome. Unfortunately, each browser has some restrictions in interpreting HTML and CSS, therefore the appearance of a website can be browser-dependent in some cases. Using correct HTML and CSS coding can reduce the variation.
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