There are some sites still use odd numbers for font sizes, but why we used "commonly" even numbers?
The point is a measurement system inherited from traditional print typography. It has had various definitions, much like the inch and foot. With the introduction of PostScript, it has been defined to be 1/72 inch. I don't recall the specific history, but the use of certain font sizes long predates computing. They continue to be used because they work, and there is no pressing need for change.
Apparently, px doesn't really stand for "pixels". Rather, it is defined as 1/96 inch. For traditional displays, which are 96 dpi, a px is equivalent to a pixel. However, for printers and high-dpi displays, it's different. Converting some common point sizes to px, we get:
- 14pt = 14*96/72 = 18.7 px.
- 12pt = 12*96/72 = 16 px.
- 10pt = 10*96/72 = 13.3 px.
The conversion results in fractional pixels that cannot be displayed accurately on standard displays, even with anti-aliasing and sub-pixel rendering tricks. So web designers may choose to round up or down, according to their preference, habit, or copying of stylesheets. (For example, 13.3 rounded up is 14, even. Rounded down is 13, odd.)
This has to do with technical dependency.
There is no "technical dependency" that affects choice of even vs odd pixel sizes.
Just like icon sizes, font sizes or any other fixed dimension in pixels are maintained at even numbers to support scaling.
There is nothing about even vs odd numbers that affects scaling. For instance, 80% of 24 = 19.2; 80% of 15 = 12. However, 75% of 24 = 18; 75% of 15 = 11.25.
... browsers cannot render pixels in decimal.
Huh? My browser works with base 10 just fine.