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It is a CSS technique which sets the style when an element changes its state. E.g. style changes when mouse hover, different styles for visited or unvisited links, etc.
It is usually not a good practice to use global variables in programming languages including JavaScript. However, you may be forced to use global variables sometimes.
HTML and XHTML are both known for writing web pages. But, HTML is extended from SGML tagging whereas XHTML is an application of Extensible Markup Language (XML).
