What defines the algorithm behind absolute and relative font sizes?
For Example: font-size: smaller or font-size: small
I know that this relative/absolute font-size declaration makes the font smaller by a certain percentage, but what is defining this percentage, and how can I manipulate it?
I'm aware that there are ways to achieve the same result in different ways. But I would love to use this in my CSS by having control over the final result.
Couldn't find any explanation yet. https://developer.mozilla.org/en-US/docs/Web/CSS/font-size
I know that this relative/absolute font-size declaration makes the font smaller by a certain percentage, ...
No it doesn't. The ratios differ. See the table at https://www.w3.org/TR/css-fonts-4/#absolute-size-mapping. Browsers may tweak the values as they see fit, on a per font basis if necessary.
how can I manipulate it?
You can't. Skip the named font sizes and just specify the pixel sizes, em/rem ratios or percentage values as you require them.