By far the worst prob for old Win98 browsers in today's web is outdated https-encryption (SSL/TLS).
Websites insisting to connect with modern TLS1.2 are completely BLOCKED, showing only error pages, usually this infamous one:
Error code: ssl_error_no_cypher_overlap
That desaster started only 1-2 years ago with Google's command to encrypt the whole public web with modern standards too, and things went downhill fast. Meanwhile half the internet is broken for old Firefox/Mozilla browsers! Amazingly, old Opera12.02 (needs KernelEx) is still holding up much better, it was amazingly advanced