Adding more infinite loop detection

js/markdown.js

 		// nodes, until no more substitutions can be made.
 		var anyChanges = false;
 		do {
+			this.checkExecutionTime();
 			anyChanges = false;
 			for (const readerTuple of this.root.readersBySubstitutePriority) {
 				/** @type {number} */
 		return nodes;
 	}
 
+	#startTime = performance.now();
+
+	/**
+	 * Checks if parsing has taken an excessive length of time. Because I'm not
+	 * fully confident in my loops yet. :)
+	 * 
+	 * @param {number} maxSeconds
+	 */
+	checkExecutionTime(maxSeconds=1.0) {
+		const elapsed = (performance.now() - this.root.#startTime) / 1000.0;
+		if (elapsed > maxSeconds) {
+			throw new Error("Markdown parsing taking too long. Infinite loop?");
+		}
+	}
+
 	/**
 	 * Mapping of reference symbols to URLs. Used by `MDReferencedLinkReader`
 	 * and `MDReferencedImageReader`.
 			return true;
 		});
 		if (match === null) return false;
+		state.checkExecutionTime();
 		let content = (plaintext)
 			? match.contentTokens.map((token) => token.original).join('')
 			: state.tokensToNodes(match.contentTokens);

php/markdown.php

 		// nodes, until no more substitutions can be made.
 		$anyChanges = false;
 		do {
+			$this->checkExecutionTime();
 			$anyChanges = false;
 			foreach ($this->root()->readersBySubstitutePriority as $readerTuple) {
 				/** @var int */