High-density monitors or Retina monitors sold by Apple are generally recognized as having a physical pixel density greater than 200 pixels per inch (PPI). This means that they have twice the pixel density of a screen resolution computer monitor. Smartphones and tablets are the main drivers for this trend due to their low cost and high pixel density, which is usually higher than 250 PPI. This means that the average person cannot see individual pixels on a high-density screen, 10-15 inches away on a smartphone or tablet, or 20+ inches on a laptop or computer screen.

The reason that CSS pixel ratio was created is because as phones screens get higher resolutions, if every device still had a CSS pixel ratio of 1 then webpages would render too small to see.
4K monitors for PCs hit the shelves in 2014, and understanding pixel density has become important when choosing products along with screen size and resolution. It is predicted that 4K will replace Full HD as the standard resolution. Of course, 4K stands for 4,000 and refers to a horizontal number of pixels around this number.

DCI 4K has twice the screen resolution of 2048 x 1080 pixels on projectors (4096 x 2160 / approx. 17: 9) and is the 4K industry's 4K resolution. UHD 4K (also called UHDTV 4K), on the other hand, is the 4K resolution of the television industry, which was defined by the International Telecommunication Union (ITU). It has twice the horizontal resolution of 1920 x 1080 pixels Full HD (3840 x 2160/16: 9).
Since the second half of the 2000s, square screens with 5: 4 and 4: 3 image formats have been in decline in the PC screen market, while 16: 9 and 16:10 widescreen screens are on the rise and becoming too established. At the same time, there was a shift from square 17 "and 19" screens to 23 "and 24" widescreen screens. There is also an active trend for 27 "or larger widescreen displays for even more comfortable environments. This transition is split between those looking for a larger workspace, 3840 x 2160 pixels (UHD 4K) or 2560 x 1440 Pixel select ( WQHD), and those looking for a screen with better visibility at a lower price, choose 1920 x 1080 pixels (Full HD). In recent years, ultra-wide screen products with even wider screens have also hit the shelves. These are super wide screen products with a 21: 9 aspect ratio.

Moreover, the world of 8K, which follows 4K and 5K, is almost there. 8K (7680 x 4320 pixels / 16: 9) compatible screen test models have appeared at video shows and events, and the transition to higher resolution and higher resolution will continue at a rapid pace. Looking at stand-alone monitors, a hot topic lately has been super high pixel density 4K high-resolution pixel monitors packed in 24- to 27-inch screen sizes. Originally, this genre only attracted the attention of some top consumers, but cheap products began to appear on the shelves one after another in 2014, so the number of regular users showing interest increased.
Computer monitors are designed based on the assumption that OS and applications will have a fixed screen density (96 ppi for Windows). The 96 dpi standard is behind this assumption, and the screen size increased with the higher resolution of the LCD panel (increased number of pixels), so it was safe to just consider that the higher the resolution (number of pixels), the larger the working area.