Bookmarklet: save webpage as clean Markdown file.

turndown.bookmarklet.js

javascript:
!function(){
	const
	base64 = x => btoa(unescape(encodeURIComponent(x))),
	onload = x => {
		if (!Readability || !TurndownService) return;
		
		const content = new Readability(document.cloneNode(true)).parse().content,
			text = new TurndownService().turndown(content) 
				+ `\n\n[original]: ${document.location.href} "${document.title}"\n`,
			link = document.createElement('a');
		link.href = `data:text/markdown;base64,${base64(text)}`;
		link.download = document.title.replace(/\s+/g,'-') + '.md';
		document.body.appendChild(link).click();
	},
	require = x => document.head.append(Object.assign(document.createElement('script'),{src:x,onload:onload}));

	require('//cdn.jsdelivr.net/npm/@mozilla/readability/Readability.min.js');
	require('//cdn.jsdelivr.net/npm/turndown/dist/turndown.min.js');
}()

Minified version:

javascript:!function(){const ll=x=>{if(!Readability||!TurndownService)return;const e=new Readability(document.cloneNode(!0)).parse().content,t=(new TurndownService).turndown(e)+`\n\n[original]: ${document.location.href} "${document.title}"\n`,d=document.createElement("a");d.href=`data:text/markdown;base64,${btoa(unescape(encodeURIComponent(t)))}`,d.download=document.title.replace(/\s+/g,"-")+".md",document.body.appendChild(d).click()},req=x=>document.head.append(Object.assign(document.createElement("script"),{src:x,onload:ll}));req("//cdn.jsdelivr.net/npm/@mozilla/readability/Readability.min.js"),req("//cdn.jsdelivr.net/npm/turndown/dist/turndown.min.js")}()