src/vs/workbench/contrib/notebook/browser/viewModel/markupCellViewModel.ts/MarkupCellViewModel/getHTML
class MarkupCellViewModel:	getHTML(): HTMLElement | null {
		if (this.cellKind === CellKind.Markup) {
			if (this._html) {
				return this._html;
			}
			const renderer = this.getMarkdownRenderer();
			const text = this.getText();

			if (text.length === 0) {
				const el = document.createElement('p');
				el.className = 'emptyMarkdownPlaceholder';
				el.innerText = nls.localize('notebook.emptyMarkdownPlaceholder', "Empty markdown cell, double click or press enter to edit.");
				this._html = el;
			} else {
				this._html = renderer.render({ value: this.getText(), isTrusted: true }, undefined, { gfm: true }).element;
			}

			return this._html;
		}
		return null;
	}