If you plan to surf on the web during a presentation, plan meticulously the navigation sequence. Students can grow fatigued and uninterested watching you click and browse, looking for the correct link. Also, if you are not the author of the pages you are planning to visit, it is a good idea to check the links the day of the presentation. Certain pages and links undergo continual editing, and may not always look the same from one visit to the next. Also make sure you've checked the pages in the browser you'll use in class.
Some pages interact differently with different browsers.
It is less risky to download the pages and run them from a disk, rather than rely on the active server. Finally, be sure to enlarge the size of the display font in the browser and minimize clutter on screen by turning off extraneous buttons bars. One of the most important considerations in using any large classroom media is instructor mobility. In particular, computers usually require you to stand close by in order to navigate. In a small, intimate, conversational setting, this is not a problem.