include 'zstd' in Accept-Encoding header when supported

… and slightly update optional dependency list

README.rst

@@ -25,10 +25,12 @@ Dependencies
 Optional
 --------
 
+- yt-dlp_ or youtube-dl_: HLS/DASH video downloads, ``ytdl`` integration
 - FFmpeg_: Pixiv Ugoira conversion
-- yt-dlp_ or youtube-dl_: Video downloads
+- mkvmerge_: Accurate Ugoira frame timecodes
 - PySocks_: SOCKS proxy support
 - brotli_ or brotlicffi_: Brotli compression support
+- zstandard_: Zstandard compression support
 - PyYAML_: YAML configuration file support
 - toml_: TOML configuration file support for Python<3.11
 - SecretStorage_: GNOME keyring passwords for ``--cookies-from-browser``
@@ -417,11 +419,13 @@ To authenticate with a ``mastodon`` instance, run *gallery-dl* with
 .. _pip:        https://pip.pypa.io/en/stable/
 .. _Requests:   https://requests.readthedocs.io/en/master/
 .. _FFmpeg:     https://www.ffmpeg.org/
+.. _mkvmerge:   https://www.matroska.org/downloads/mkvtoolnix.html
 .. _yt-dlp:     https://github.com/yt-dlp/yt-dlp
 .. _youtube-dl: https://ytdl-org.github.io/youtube-dl/
 .. _PySocks:    https://pypi.org/project/PySocks/
 .. _brotli:     https://github.com/google/brotli
 .. _brotlicffi: https://github.com/python-hyper/brotlicffi
+.. _zstandard:  https://github.com/indygreg/python-zstandard
 .. _PyYAML:     https://pyyaml.org/
 .. _toml:       https://pypi.org/project/toml/
 .. _SecretStorage: https://pypi.org/project/SecretStorage/

gallery_dl/extractor/common.py

@@ -390,6 +390,8 @@ class Extractor():
             headers["Accept-Encoding"] = "gzip, deflate, br"
         else:
             headers["Accept-Encoding"] = "gzip, deflate"
+        if ZSTD:
+            headers["Accept-Encoding"] += ", zstd"
 
         referer = self.config("referer", self.referer)
         if referer:
@@ -990,6 +992,12 @@ try:
 except AttributeError:
     BROTLI = False
 
+# detect zstandard support
+try:
+    ZSTD = urllib3.response.HAS_ZSTD
+except AttributeError:
+    ZSTD = False
+
 # set (urllib3) warnings filter
 action = config.get((), "warnings", "default")
 if action: