HTML is based on the principle that the browser should determine the appearance of the page. The page designer has little control over how the text is rendered by browsers, but instead gives instructions and hopes that the browsers do the right thing. Browsers typically perform word wrap and have available several fonts. Which fonts are actually used is determined by browser settings. Thus the page designer cannot know where line breaks will occur in text, how graphical images will be aligned, and so on.
Netscape extensions to HTML return some control to the page designer, who can center text, provide background patterns, and display tables. Not all browsers support these non-standard extensions.