They don't want to change their default fonts or monitor resolution just to see your site. The same goes for software. Like most software, the newest browsers have features that aren't supported by the older browsers, and also like most software, there are those who refuse to upgrade. If you want your page to be displayed properly in older browsers, you need to be careful about using the more recent features. Because there are so many different software packages, out there, this section has been intentionally generalized.
Programs often do similar things, just in different ways.
You may have to dig a bit to find the right functions in a particular program, whether it's a graphics or HTML editor. Please let us know if this information is not specific enough. A final note It would be impossible to give a comprehensive course on web site design in just a few hours.