The following is from the: Techencyclopedia Web site / techweb.com
WYSIWYG:
(What You See Is What You Get) Pronounced "wizzy-wig." It refers to displaying text and graphics on screen the same as they will print. To have WYSIWYG text, there must be an equivalent screen font for each printer font used.
The screen and printed results may look the same, but, in fact, it is impossible
to get an exact representation because screen and printer resolutions do not
match. The typical monitor may display 75 dpi, whereas a low-end desktop printer
prints 300 dpi. That means the screen produces approximately 5,000 dots per
square inch compared to the printer's 90,000.
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As far as they pertain to Web design, the WYSIWYG editor would allow the Web author to design Web sites in a graphical environment. The GUI (graphical user interface) would allow the Web author to see the way Web page content such as text and images would be displayed in the clients browser while the author is sill designing the Web pages within the WYSIWYG editor.
The "What You Get" part of the equation would be met when the Web authors creation is viewed in a browser and displays in the same way it displayed in the WYSIWYG application used to created it. Which is similar to the way that the printer version of a text document is the proof of the desktop publishing WYSIWYG editor power.
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Web Authoring Software:
A Web site development system that allows Web pages to be visually created like a desktop publishing program. It generates the required HTML code for the pages and is able to switch back and forth (in varying degrees) between the page layout and the HTML. At a high level, the software is judged by its GUI tools used for designing the page. At a low level, the clarity of HTML code that is generated determines how easily people can modify and maintain the site.
Comprehensive products can read an entire Web site and display it as a graphical hierarchy of pages, providing a way to manage existing sites.
Web Development Software:
Software used to develop Web sites. Although often synonymous with "Web authoring software," it implies a more programming-oriented set of tools for linking pages to databases and other software components. It generally includes an HTML editor.