• The bookmark, a so-called bookmarklet, runs a special script when you open it, instead of opening a website. See How-To Geek's beginner guide to learn more.
• Open the bookmarklet directly on a TVthek video page. Running the bookmarlet outside the TVthek will not work.
• To create a bookmark on mobile, copy the link, create a bookmark of this page, and change the URL to the copied link (which starts with javascript:)
• You can show/hide the bookmark bar on desktop Chrome using Ctrl+Shift+B
• You can also copy the link directly into the address bar for one-time use. Your browser might omit the javascript: part for security reasons. In this case, type it out manually, because otherwise, the script will not run.
• For convenient copy-pasting, here is the bookmarklet link:
  

  javascript:(function(){s=document.createElement('script');s.type='text/javascript';s.src='https://sk22.github.io/orf-downloader/downloader.js?v='+(Date.now());document.body.appendChild(s);})();

• You can look at the script's source code in the GitHub repository: sk22/orf-downloader

quick start guide

architectural limitations

• Single chunks may appear glitchy. TVthek splits up their full videos into chunks of 10 seconds, as it uses HTTP Live Streaming. Since the individual chunks are not meant to be played alone, but as a playlist (as defined in the chunklist files), the beginnings and endings will appear glitchy. Software like HandBrake should be able to fix those problems.
• TS files cannot be shared on social media. Most social media websites like Twitter do not accept .ts video files. You will have to convert it to a compatible format using software like HandBrake, ffmpeg, the VLC media player, or an online converter like cloudconvert. (This information is only half true, but I don't know that much about video encoding, muxing, etc.) You can, however, upload your downloaded TS file directly to YouTube (but, regarding copyright, you probably shouldn't).
• The tool doesn't convert the files automatically. This tool runs in your browser and does not communicate with any third-party server while downloading videos from TVthek. Since TVthek only provides the videos in the TS format, all this tool can provide is files in this format. There is no encoding whatsoever going on; the longer videos are generated by simply concatenating the single media_n.ts files.

alternatives

• youtube-dl: A download utility for the command line that can extract media from YouTube, and a wide variety of websites, including ORF stations. The software is most likely more robust than mine, downloaded videos are automatically muxed as mp4 files via ffmpeg, but users need to (know how to) use a command line to operate it. Also, I don't know a way to only download a single section - youtube-dl will always output multiple video files for each section.
• MediathekView: A desktop program for viewing and downloading media from (german-speaking) TV channels. Note that there are necessary tweaks to be made in order to use MediathekView with ORF.
• JDownloader: A well-known download manager with loads of features and compatible websites. Using its TVthek plugin, it can also download videos from there. Note that JDownloader might be a bit overpowered for a task as little as downloading a single video.

related links

• Karl Voit's article about using MediathekView to download media from ORF TVthek. This includes the necessary User-Agent tweaks.
• Reddit post for this tool on r/Austria. Alternatives are discussed in the comments.