When I do:

background-size 100%

in the Stylus, the CSS output by Responsive Pixels is:

background-size: 100% 1px 1px; background-size: 100%;

Not sure where the 1px 1px comes from, but any % value should be ignored by Responsive Pixels.

EDIT: It does this on other values as well. For example:

table th padding 5px 10px

is rendered as:

padding: 5px 10px 1px 1px; /* Make sure that the rule is written out only once in case there is no rem substitution */ padding: 0.3125rem 0.625rem;