>https://github.com/bigskysoftware/htmx/blob/1242977d11bebe56...

This seems to be a scenario where switch/case blocks could make the elif-trees a bit easier to read.

Also, the code could use some care about not going so deep into the Vs:

    // request headers
    if (requestAttrValues.noHeaders) {
    // ignore all headers
    } else {
      for (const header in headers) {
        if (headers.hasOwnProperty(header)) {
          const headerValue = headers[header]
          safelySetHeaderValue(xhr, header, headerValue)
        }
      }
    }

could just be:

    // request headers
    if (!requestAttrValues.noHeaders) {
      Object.keys(headers)
        .filter(hdr => headers.hasOwnProperty(hdr)) 
        .forEach(hdr => safelySetHeaderValue(xhr, hdr, headers[hdr]));
    }

Not even sure if that hasOwnProp check is needed, unless header keys are explicitly set to undef.