Markdown content negotiation

              DOMAIN="https://example.com"
curl -sSL -D /tmp/md.h -o /tmp/md.body -H 'Accept: text/markdown' "$DOMAIN/"; grep -iE '^(HTTP/|content-type|vary)' /tmp/md.h; head -3 /tmp/md.body
            

              // server.js — serve a prebuilt markdown twin on Accept: text/markdown (RFC 9110).
import express from 'express';
import { readFile } from 'node:fs/promises';

const app = express();

app.use(async (req, res, next) => {
  if (req.method !== 'GET') return next();
  res.vary('Accept'); // both representations share one URL, so caches must key on Accept
  if (req.accepts(['text/html', 'text/markdown']) !== 'text/markdown') return next();
  const file = `./public${req.path.replace(/\/$/, '/index')}.md`;
  try {
    res.type('text/markdown').send(await readFile(file, 'utf8'));
  } catch {
    next(); // no markdown twin for this path: fall back to the HTML document
  }
});

app.use(express.static('public'));
app.listen(3000);