HTML files are Plain text (ascii) format, created using any text editor.
Many WYSIWYG HTML editors exist, e.g. Claris Home Page, Macromedia Dreamweaver. Netscape Communicator also has a built in editor.
It is however recommended that you at least learn the basics of HTML tagging before you dive into WYSIWYG editing as this mask the programming issues and ultimately hinders the learning process.