The units pixel and point have meaning in areas other than web sites. For printed documents 72 points = 1 inch. Tools like Photoshop edit image in actual pixel unit. When a photo is printed at 300dpi, a 300 pixel x 300 pixel image will print 1" x 1".

However, for purposes of web site design the difficulty is the wide variations in pixel density. For example, in theory, most standards screens us 72 PPI, the Phone X uses 458 PPI, and the Samsung Galaxy S8 uses 568 PPI. However, various up-sizing and downsizing of pixel mappings occur between and web source and the final device. To add further confusion, there a em (font size relative to parent) and rem (root font size) units used in web site design. In practice most browsers set 1 rem = 16px. Generally 12pt = 16px. Because these units are relative, it is often useful to know compatible ratios at design time.