The current syntax of margin-trim is:

margin-trim: none | all | in-flow;

Assuming that floats and in-flow content don't need distinct trimming values (see #3256 (comment)), and that authors probably want to be able to control which sides are trimmed (with block-axis and block-start being likely, and sometimes all sides), suggest to change the syntax to:

margin-trim: none | block | inline | [ inline-start || inline-end || block-start || block-end ];