They often use a telnet or TN3270 interface, which may require an add-on to your web browser. The simplest rule is to view all compiled data as clues, not facts. Images of original documents are completely reliable but subject to individual interpretation. Besides these, the quality of electronic data is often hard to judge. Most providers of pedigree data exercise little or no quality control.
Web sites run the gamut from highly reliable to fictional.
Often a web page has the purpose of drawing other researchers into contact with the page's author. In this case the author's data may be more detailed and better documented than the web page. Look for quality of sources in the material, completeness of facts about individuals, and obvious problems then form your own judgement. It may take a few tries to get the content right. There are plenty of good books and online tutorials on building good web sites - consider using pedigree pages as part of a larger package.