Yes, it is a bug (in my modifications, not in tufte.css itself), or at least an unintended side effect. It is on my list of things to revisit but I am prioritizing writing posts instead of futzing around with the styles.
The general layout of Tufte.css really wants to be larger in the horizontal direction to fit in the side notes. This causes problems for people who read the page in a narrow window or on cell phones.
The general layout of Tufte.css really wants to be larger in the horizontal direction to fit in the side notes. This causes problems for people who read the page in a narrow window or on cell phones.