Is it 72 or 96 dpi? Baseline 1:1 graphics for web.

It's PPI, not DPI.

For web, it's historically been neither 72 ppi or 96 ppi – it's the total absence of resolution metadata altogether as it's irrelevant for monitor/device display purposes. A 7 ppi image displays in a browser the same size as a 7000 ppi file of the same pixel dimensions. Some software displays the absence of this metadata as 72ppi (Mac OS, Adobe apps and other apps) or 96 ppi (Windows OS and other apps).

As Adobe Photoshop requires a resolution value to consistently size smart objects, Adobe are now including the source document resolution metadata via Export As (which it didn't historically do). Export > Save for Web (Legacy) allowed the inclusion or exclusion of this metadata in supported formats.

And 300 ppi for print is just a general historical rule of thumb, best case scenario of AM/Halftone screening when running a 150 lpi (lines per inch) screen when viewed at arm's length. The historical formula is 1.5-2.0 x the linescreen when used at 100% size, so 225-300 ppi has always been acceptable and that's why you hear 300 ppi so often (150 lpi x2 = 300 ppi).