The min-width property accept only values <length> | <percentage> | inherit. to reset better set to 0.
Properties documentation: https://www.w3.org/TR/CSS2/visudet.html#min-max-widths

This error generates an invalid style, and, therefore, this value is set to 760px by: https://github.com/edx/edx-platform/blob/master/lms/static/sass/course/courseware/_courseware.scss#L50