The important thing to realise about HTML is that it was only intended to describe the structure of the text, not the layout. For some, but not all, paired tags the closing tag is optional. However, it is good practice to use a closing tag where one exists. The case in which the tags are entered does not matter. Many HTML editors offer the facility to colour tags, as used in the examples here.
Some tags have attributes which modify the way in which they work.
Strict HTML 4.0 uses style sheets to specify the appearance of a page but not all browsers understand style sheets and there is an inconsistency of interpretation amongst those that do. Therefore, transitional HTML 4 is described here. There are also many sites offering HTML tutorials. The basic page Each page contains the following tags in addition to any text and formatting. It appears in the top bar of the browser, is saved in bookmarks and is looked at by search engines.