A few comments:

1. fetch already returns a promise, which means this:

   new Promise((resolve, reject) => { return fetch(url).then(response => { if (response.ok) { resolve(response) } else { reject(new Error('error')) } }, error => { reject(new Error(error.message)) }) })

Is pretty much the same as:

fetch(url).then(response => {
  if (response.ok) {
    return response
  }
  return Promise.reject(Error('error'))
}).catch(error => {
  return Promise.reject(Error(error.message))
})

2. Keeping this in mind you can simplify your code with early returns and fewer branches:

   function get(url) { return fetch(url).then(response => { if (response.ok) { const contentType = response.headers.get('Content-Type') || '';

      if (contentType.includes('application/json')) {
        return response.json().catch(error => {
          return Promise.reject(new ResponseError('Invalid JSON: ' + error.message));
        });
      }
   
      if (contentType.includes('text/html')) {
        return response.text().then(html => {
          return {
            page_type: 'generic',
            html: html
          };
        }).catch(error => {
          return Promise.reject(new ResponseError('HTML error: ' + error.message));
        })
      }
   
      return Promise.reject(new ResponseError('Invalid content type: ' + contentType));
    }
   
    if (response.status == 404) {
      return Promise.reject(new NotFoundError('Page not found: ' + url));
    }
   
    return Promise.reject(new HttpError('HTTP error: ' + response.status));
   

   }); }

A few comments:

1. fetch already returns a promise, which means this:

   new Promise((resolve, reject) => { return fetch(url).then(response => { if (response.ok) { resolve(response) } else { reject(new Error('error')) } }, error => { reject(new Error(error.message)) }) })

Is pretty much the same as:

fetch(url).then(response => {
  if (response.ok) {
    return response
  }
  return Promise.reject(Error('error'))
}).catch(error => {
  return Promise.reject(Error(error.message))
})

2. Keeping this in mind you can simplify your code with early returns and fewer branches:

   function get(url) { return fetch(url).then(response => { if (response.ok) { const contentType = response.headers.get('Content-Type') || '';

      if (contentType.includes('application/json')) {
        return response.json().catch(error => {
          return Promise.reject(new ResponseError('Invalid JSON: ' + error.message));
        });
      }
   
      if (contentType.includes('text/html')) {
        return response.text().then(html => {
          return {
            page_type: 'generic',
            html: html
          };
        }).catch(error => {
          return Promise.reject(new ResponseError('HTML error: ' + error.message));
        })
      }
   
      return Promise.reject(new ResponseError('Invalid content type: ' + contentType));
    }
   
    if (response.status == 404) {
      return Promise.reject(new NotFoundError('Page not found: ' + url));
    }
   
    return Promise.reject(new HttpError('HTTP error: ' + response.status));
   

   }).catch(error => { return Promise.reject(new NetworkError(error.message)); }); }