[Tex2002ans]

Originally Posted by Enterio

Is there any easy way to edit the CSS to force the Kindle to show figures as old-style figures?

Not really.

You can add the CSS there, and just hope future devices can render Oldstyle Figures (it won't hurt... any device that doesn't understand the CSS will just ignore it).

Here's the Oldstyle Figures code:

font-variant-numeric: oldstyle-nums;   /* high-level property          */
-moz-font-feature-settings: "onum";    /* low-level (old Firefox)      */
-webkit-font-feature-settings: "onum"; /* low-level (old Webkit)       */
font-feature-settings: "onum" on;      /* low-level (all new browsers) */

that I grabbed from Adobe's fantastic article: "Syntax for OpenType features in CSS".

Side Note: More of this was discussed in technical detail in my 2019 post in "Turn off ligatures (temporarily)". Also see the references to Microsoft's + Mozilla's OpenType documentation.

(Also, check out the rest of that thread... there's a ton of great information in there.)

Originally Posted by Enterio

This isn’t very good, and I wouldn’t get that effect on other ebook readers. Is there any solution available? The CSS code—

This code is more advanced CSS3, so many older devices won't support this. Also, it has to have OpenType fonts which have such alternates available.

Originally Posted by Notjohn

What's a "figure"? Indeed, what's an old-style number? [...] I can't picture a number of that sort.

Take a look at the post above, where I link to multiple articles with graphics showing what these advanced settings enable.

Here's Mozilla's "OpenType Font Features Guide" for example. They also have brief descriptions of what each setting is used for.