Script levels in MathML are about managing the font-size in subexpressions (like scripts and fractions). This example illustrates and describes some of the concepts/implications. The method for doing this is not quite as simple as inheriting, so the MathML-Core spec introduces a new
Depending on context (exponents, fractions, etc) and particulars of rendering style (compact/inline), the font-size of of parts of expressions scale down at different and fairly well established rates - for example, the scaling factor adjustment of a superscript is not the same as that of a root's prescript. The example below shows several contextual scaling variations.
The common rules derive from TexBook, but they are subtle and deserve to be exposed for tweaking and extension by authors. To this end, MathML adds a new
scriptlevel function for CSS Fonts Module Level 4 which allows authors to express that the value should be automatically derived (default), or explicitly set to a specific level or that it should be the current level + some integer with rules defined in section 4.5 of the spec