This line breaks with res.headers.raw() when using rdf-fetch-lite in a browser.
https://github.com/rdf-ext/rdf-fetch-lite/blob/9e574b83aa7d8d80c9ee41feb7141ee01de61dca/lib/patch-response.js#L20
My guess is that the method is not implemented in whatwg-fetch.

When fetching JSON-LD context, no accept header is set, which for example doesn't work at http://ld.stadt-zuerich.ch/context/flaechen.context.json

I'm not entirely sure what the correct thing to do should be (setting explicit options, pass through the options of the original request). In any case, setting {headers: {accept: 'application/ld+json'}} as fetch options works.

https://github.com/rdf-ext/rdf-fetch-lite/blob/9e574b83aa7d8d80c9ee41feb7141ee01de61dca/lib/patch-response.js#L31

I'm using rdf-store-sparql, and it uses this library.
When i'm doing client.selectQuery(sparqlQuery, {headers:{'Accept':'application/sparql-results+json'}}), the header just disappears before the request is send

The line delete context.options.headers.Accept in patch-request.js deletes the right header and leaves me with text/turtle instead.

I've hardcoded a nasty fix

  if (context.options.accept === 'application/sparql-results+json'){
    delete context.options.headers.accept
  } else {
    delete context.options.headers.Accept
  }