HTML documents are in plain (also known as ASCII) text format and can be created using any text editor (e.g., Emacs or vi on UNIX machines). A couple of Web browsers (tkWWW for X Window System machines and CERN's Web browser for NeXT computers) include rudimentary HTML editors in a WYSIWYG environment. There are also some WYSIWIG editors available now (e.g. HotMetal for Sun Sparcstations, HTML Edit for Macintoshes [NOTE: HTML.edit is not WYSIWYG -- ma]). You may wish to try one of them first before delving into the details of HTML.
You can preview a document in progress with NCSA Mosaic (and some other Web browsers). Open it with the Open Local command under the File menu.
After you edit the source HTML file, save the changes. Return to NCSA Mosaic and Reload the document. The changes are reflected in the on-screen display.