19 October 2021
Almost all modern websites are now inaccessible to early Mac OS internet browsers and likely won't load anything. This is due to modern web standards advancing which were never present within these browsers at the time they were released. Another problem is that modern websites are typically served via the HTTPS protocol which is incompatible with most browsers, rather than the less secure HTTP protocol. For further reading, see HTTPS and why it's a problem for early Mac OS.
Websites that have been designed with older browsers in mind, or sites that have remained live since before the early 2000s are accessible on these browsers.
There has been an effort to compile a directory of modern and existing websites that are accessible to older web browsers. See the Sloop Web Directory.
Classilla was the last web browser that received recent updates and was maintained regularly until 2021 by Cameron Kaiser. The only downside to using Classilla is that it is resource hungry and requires later Mac OS versions. Classilla requires a G3 Processor, 64MB ram (minimum) and Mac OS 8.6.
Internet Explorer 5 is the next best browser for older Mac OS systems. During it's time IE5 had excellent support of web standards and was the go-to choice for internet users. IE5 uses minimal system resources and runs on Systems 7 - 9.2.2.