I use Roytam1's Firefox 45 ESR SSE on my overclocked Pentium 3 Tualatin RDD, coupled with the UOC Patch. As a more lightweight alternative there is K-Meleon 76 Goanna, always developed by Roytam1, and New Moon 27. I keep those three installed on the RDD, and also thanks to the patch, they run quite fast.
A friend of mine is using K-Meleon 76 Goanna alongside the UOC Patch on a 500MHz Celeron Mendocino with only 256MB of RAM, and it runs quite well on Windows 2000.
You can find more information about these browsers at the link below: https://msfn.org/board/topic/177125-my-build- … -targetting-xp/
While the UOC Patch can be found right here, in the Software section of this forum.
All the browsers above support HTML5, CSS and the modern security protocols (including TLS 1.3). On a very old CPU without the SSE instructions, you can use New Moon 27 IA-32, which runs on an original Pentium and Pentium II.
My Retro Daily Driver: Pentium !!!-S 1.7GHz | 2GB PC166 ECC SDRAM | Gecube Radeon X1950 Pro 256MB OC | 128GB Lite-On SSD + 500GB WD Blue SSD | Creative Sound Blaster Live! CT4620 | Windows XP Professional SP3