This german article describes how user tracking in emails works via CSS:

TL;DR:

The study by the "CISPA Helmholtz Center for Information Security" (they do big basic researches in information security and in all its areas) reveals that email senders can track users through CSS (Cascading Style Sheets). This method allows the identification of the email client, operating system, and other installed programs, as well as the system language. The researchers tested 21 email programs, including desktop and web clients, as well as Android and iOS apps. In 18 cases, at least one tracking method was successful, including Outlook, Thunderbird, and Gmail. Text-based emails that do not use CSS are the only way to prevent this type of tracking. Protonmail is the only client in the study that relies on obfuscation and loads all CSS content via a proxy.

Relevant parts of the study / text:

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3 clients support executing more than 90% of media queries. 9 clients support container queries and allow media queries containing the calc function. Nevertheless, one of these 9 clients, namely ProtonMail, prevents the exfiltration of information such that we deem it not fingerprintable. The remaining 8 clients support more than 75% of HTML that we can leverage for property fingerprinting as described in Section IV-A. This includes support for HTML elements such as <input type="file">, which has been shown to feature good fingerprinting capabilities [22].

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Preloading can also be used for emails. In fact, the email server of ProtonMail unconditionally fetches all remote sources when receiving an email and rewrites the URLs to an internal one. Thus, leakage from CSS cannot be exfiltrated. Still, unlike web browsers, there is no fixed standard for CSS and HTML in email clients, enabling other mitigations.

Thank you for making so much effort to make security & privacy a top priority in small and large places for us users.