New source found from dndbeyond.com

ddb_main/node_modules/@mui/utils/esm/capitalize/capitalize.js

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+import _formatMuiErrorMessage from "@mui/utils/formatMuiErrorMessage";
+// It should to be noted that this function isn't equivalent to `text-transform: capitalize`.
+//
+// A strict capitalization should uppercase the first letter of each word in the sentence.
+// We only handle the first word.
+export default function capitalize(string) {
+  if (typeof string !== 'string') {
+    throw new Error(process.env.NODE_ENV !== "production" ? `MUI: \`capitalize(string)\` expects a string argument.` : _formatMuiErrorMessage(7));
+  }
+  return string.charAt(0).toUpperCase() + string.slice(1);
+}