Mains voltage PCB design
When working with line voltage there's some key points that come up:


Insulation / isolation between mains and low-voltage
Creepage / clearance on the board
Fire retardant materials (that's where the 'FR' in FR2 comes from)
Safety approved components


The main standards for the board that apply are:


IPC-2221A, "Generic Standard on Printed Board Circuit"
IEC-60950-1 (EU and many others)
UL 60950-1 (USA, generally harmonizes with IEC-60950-1)


Some more info:

https://resources.altium.com/pcb-design-blog/high-voltage-pcb-design-creepage-and-clearance-distance

And, there is a new standard replacing IEC-60950-1, which uses hazards-based methodology, called IEC-62368-1. This takes effect on Dec. 20 2020.

More here: https://www.cui.com/blog/the-latest-on-iec-62368-1