Although desktop publishing and Web design have a common ancestry, they aren't the same. Yes, there are certain similarities — such as text, graphics, color, page composition, and the need for clear navigation -- but Web design has its own set of challenges and design parameters.
Writing and reading on-screen differs from print so typography online has its own idiosyncrasies. A font that looks great on paper may be much harder to read on-screen. And unless the font is used in a graphic, there's a strong chance that visitors to your Web page aren't going to all see the same font -- either because they don't have it installed or they use Web browser preferences that override your font choices. Those are just some of the differences between text in print and text on the Web.
Commercial printing processes are typically done in CMYK or uses Pantone spot colors or other print-friendly color specifications. On the Web, color is RGB. And then you'll also need to contend with browser-safe color schemes — maybe. The use of color in typography also differs, in part because of readability differences on-screen.
Although Web pages may use some navigational elements derived from print, such as table of contents, navigating through the interconnected pages of a Web site isn't the same as the usually linear navigation of the pages of a book. Although you can use columns in Web design, they aren't used in the same way as columns of text in, say, a magazine or newspaper.
In print design, page layouts are static designs. Once it is printed, everyone viewing the page sees the graphics in the same place, the text columns in the same size, and the piece of paper it's printed on doesn't change size or shape each time someone picks up the paper. Web pages are more fluid, more dynamic, and the reader has more control over how they view pages.