"Subscribe" bookmarklet for NewsBlur. Copy and paste the code into the "Location" field of a new bookmark.

subscribe.js

javascript: (function () {
  /* change this for other services */
  const SubscriptionUrlPrefix = "https://www.newsblur.com/?url=";

  const FeedQuerySelector = [
    'link[rel~="alternate"][type="application/atom+xml"]',
    'link[rel~="alternate"][type="application/rss+xml"]',
  ].join(", ");

  const YouTubeChannel = /^https:\/\/www.youtube.com\/channel\/([A-Za-z0-9_\-]+)$/;
  const YouTubeChannelShort = /^https:\/\/www.youtube.com\/@([A-Za-z0-9_\-]+)$/;
  const YouTubePlaylist = /^https:\/\/www.youtube.com\/playlist\?list=([A-Za-z0-9_\-]+)$/;

  const feedUrl = (function () {
    let match;
    if (
      document.documentElement.nodeName === "feed" ||
      document.body.childNodes[0].id == "webkit-xml-viewer-source-xml"
    ) {
      return document.location.href;
    } else if ((match = YouTubeChannel.exec(document.location.href))) {
      const channelId = match[1];
      return `https://www.youtube.com/feeds/videos.xml?channel_id=${channelId}`;
    } else if ((match = YouTubeChannelShort.exec(document.location.href))) {
      const canonicalUrl = document.querySelector("link[rel=canonical]").href;
      match = YouTubeChannel.exec(canonicalUrl);
      const channelId = match[1];
      return `https://www.youtube.com/feeds/videos.xml?channel_id=${channelId}`;
    } else if ((match = YouTubePlaylist.exec(document.location.href))) {
      const playlistId = match[1];
      return `https://www.youtube.com/feeds/videos.xml?playlist_id=${playlistId}`;
    } else {
      const element = document.querySelector(FeedQuerySelector);
      return element ? element.href : null;
    }
  })();

  if (!feedUrl) {
    alert("No feed found.");
    return;
  }

  const url = SubscriptionUrlPrefix + encodeURIComponent(feedUrl);
  window.open(url, "_top");
})();