"PCjs is an IBM PC simulator written entirely in JavaScript. The configuration below specifies a clock speed of 4.77Mhz, with 64Kb of RAM and a monochrome display, using the original IBM PC Model 5150 ROM BIOS and MDA font ROM. For greater control, there's a configuration with a Control Panel and the PCjs Debugger."
In case, you find the need to change the DNS server for your entire network on your router or set it individually on a PC or other device then have a look on this tutorial.