[hobnail]

Don't use this: "Tufte CSS uses #fffff8 and #111111 because they are nearly indistinguishable from their ‘pure’ cousins, but dial down the harsh contrast." So delete lines 53 and 54.

On an ereader the colors black and white are the only safe colors to use. You could use grays, but sparingly, e.g., for rules, and they may wash out and disappear. Instead of specifying black for color use "inherit", and don't specify a background color; that's because the user can change the setting to make the ereader display white text on a black background for night mode.

Line 55, max-width, is likely also not a good idea for an ereader. Delete it.

Likewise line 48; let the user set the width on their ereader. Same with line 51. Delete those.

Delete line 44; again, let the user choose on their ereader.

What I have for the body tag are:
border: 0;
font-style: normal;
hyphens: auto;
margin: 0;
padding: 0;
width: auto;

I'm not a fan of spacing above or below paragraphs so what I have for the p tag is
margin: 0;
padding: 0;
text-indent: 2em;

(I like a hefty indent.)

Get rid of the embedded fonts. Some of us have font preferences and have dodgy eyes.

Figure out which of the tags he's using can be used in which version of epub or kindle. For epub some of them can't be used with epub 2.