What is the appetite for changing from using custom HTML tags to instead using tags from an XML namespace and prefix? This will avoid IDE / parser warnings, and allow (if desired) for a DTD to help with auto-completion in IDEs.
The following example uses no more characters in the tag than the current turbo-frame, at the expense of an XML namespace declaration in the html tag:
<html xmlns="http://www.w3.org/1999/xhtml"
xmlns:turbo="https://turbo.hotwire.dev"
lang="en">
<body>
...
<turbo:frame id="the_frame">
<p>Hello, <span>world</span>!</p>
</turbo:frame>
</body>
</html>
For correctness, this would require the turbo.js to inspect the html element for an xmlns with the appropriate URL (e.g. https://turbo.hotwire.dev) and extract the nominated prefix (e.g. turbo) and use that value in the logic where it currently looks for elements called turbo-frame. Each loaded page could (although normally wouldn’t) have a different prefix.
Nonetheless, an initial implementation need not do that work and simply assume a turbo namespace prefix is used in the xmlns declaration. The existing implementation could even fall-back to turbo-frame when no xmlns was specified (for “legacy” code).