Browser Snippet to quickly get a download link for a video displayed on current page. Only tested on a couple sites. Targets 'src' attribute on <video> element or <source> element.

snippet-video-download-link.js

/**
* Create a download link for video displayed on current page and 
* put the link into either window.SELECTOR if set, or body, or if
* site is reconized by script it should add download link under the 
* video.
*/ 
(function(decoratee) {

  // Tries to get and display a link
  attemptToGetAndRenderDownloadLink();
  
  
  // Creates the download link
  function createLink(to, download = false) {
    const a = document.createElement('a');
    const s = document.createElement('strong');
    a.setAttribute('download', download);
    a.setAttribute('href', to);
    s.append(
      document.createTextNode('-Download Link-')
    );
    a.appendChild(s);

    return a;
  }

  // Creates an element that holds all elems as sibling elements.
  function siblings(...elems) {
    const frag = document.createDocumentFragment();
    elems.map(elem => frag.appendChild(elem));

    return frag;
  }

  // Where should the download link be displayed?
  function defaultSelector({location:{origin=''}, SELECTOR:custom}=window) {

    if ( custom ) {
      return custom
    }
    if (origin.match(/egghead\.io/)) {
      return '.description-text-holder'
    }
    if (origin.match(/(----\.io)|(------\.com)/)) {
      return '#reader-content .panel p'
    }

    // defaultSelector
    return 'body'
  }


  // Tries to get the video link from video element then source element.
  function getVideoLink() {
    const video = document.querySelector('video');
    let link;
    if (!video) {
      console.warn('No <video> element on the page');
      return void(link);
    }

    link = video.getAttribute('src');

    if (video.querySelector && video.querySelector('source')) {
      const source = video.querySelector('source');
      
      if (source && source.getAttribute('src')) {
        link = source.getAttribute('src');
      }
    }
    return link;
  }

  
  // "main" - try to find src, make link, add to current webpage
  function attemptToGetAndRenderDownloadLink() {

    const link = getVideoLink();   
    if (!link) {
      console.warn('Could not find a link for video.');
      return void(link)
    } 
    else {
      console.info(`Creating a link to download video @${link}`);
    }

    const theDownloadLink = createLink(link, true, 'hello');
    const selector = defaultSelector(decoratee);
    const displayElem = siblings(
      document.createElement('br')
      , theDownloadLink
    );

    document
      .querySelector(selector)
      .appendChild(displayElem);
    
    return true
  }

  // Add the functions as methods to window or passed in decoratee
  // object. This won't clobber anything.
  decoratee.attemptToGetAndRenderDownloadLink = decoratee.attemptToGetAndRenderDownloadLink || attemptToGetAndRenderDownloadLink;
  decoratee.getVideoLink = decoratee.getVideoLink || getVideoLink;
  decoratee.siblings = decoratee.siblings || siblings;
  decoratee.createLink = decoratee.createLink || createLink
}(window || {}))