<b>bold</b> produces the output: bold ;
<i>italics</i> produces the output: italics.
Because the same document may be viewed by many different `browsers', of very different abilities, HTML allows you to mark titles, paragraphs, texts, and etc, and then leaves the interpretation of these marked elements up to the browser. For example one browser may indent the beginning of a paragraph, while another may only leave a blank line.
Use the View -- Source feature under the Netscape's menu bar to see the HTML source of any document viewed with Mosaic for X.