extensions/markdown-language-features/preview-src/loading.ts/StyleLoadingMonitor/constructor
class StyleLoadingMonitor:	constructor() {
		const onStyleLoadError = (event: any) => {
			const source = event.target.dataset.source;
			this.unloadedStyles.push(source);
		};

		window.addEventListener('DOMContentLoaded', () => {
			for (const link of document.getElementsByClassName('code-user-style') as HTMLCollectionOf<HTMLElement>) {
				if (link.dataset.source) {
					link.onerror = onStyleLoadError;
				}
			}
		});

		window.addEventListener('load', () => {
			if (!this.unloadedStyles.length) {
				return;
			}
			this.finishedLoading = true;
			if (this.poster) {
				this.poster.postMessage('previewStyleLoadError', { unloadedStyles: this.unloadedStyles });
			}
		});
	}