The rsi prefix looks like it's supposed to be a namespace. Does the routine that injects this code also provide an actual namespace URL to go with it?
If you have info like that, stick the namespace info in the start tag for the HTML, and use it also in the CSS.
<!DOCTYPE html>
<!-- use the namespace attribute -->
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:rsi="(stuff)" xml:lang="en">
<head>
<title>Test rsi namespace</title>
<style>
@namespace rsi '(stuff)'; /* define same namespace in the CSS */
span {color:red} /* your original code here */
rsi|span {color:inherit} /* namespaced span needs styles reverted */
</style>
</head>
<body>
<span>This is a normal span</span>
<rsi:span>
<rsi:span>
<rsi:span>Name </rsi:span>
<rsi:span>that </rsi:span>
<rsi:span>tune</rsi:span>
</rsi:span>
</rsi:span>
</body>
</html>
As you can see, the syntax for using namespaces is different in the CSS than in the markup. See, for instance, @namespaces - css on MDN.
Edit: Unfortunately, this works in IE only if the file is a real XHTML file. So the snippet here will look like it fails, but if you copy it and paste it in a file with .xhtml for a prefix, it will run as desired.
Firefox and Chrome are good with both HTML or XHTML.