If you've created a file while reading this, you probably want to see what it looks like when displayed. Graphical editors let you specify fonts and layout while you are editing, but these may not match the various ways in which browsers interpret the markup.
Run your browser and use the Open Local function to display your file. If you're using a windowing system, you can have the browser display in one window and your editor in another, and use the browser's Reload function to redisplay the file each time you make changes (don't forget you'll have to save the file in your editor before trying to load it in the browser).
Using a character system, you'll have to save the file and exit from your editor, then run a browser as a separate command and open the file for display. If you're using Emacs in character mode, however, you can split the screen into two text windows and run the w3-mode browser in one half and the psgml-mode editor in the other half.