diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..a37894e
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,61 @@
+# Drupal git normalization
+# @see https://www.kernel.org/pub/software/scm/git/docs/gitattributes.html
+# @see https://www.drupal.org/node/1542048
+
+# Normally these settings would be done with macro attributes for improved
+# readability and easier maintenance. However macros can only be defined at the
+# repository root directory. Drupal avoids making any assumptions about where it
+# is installed.
+
+# Define text file attributes.
+# - Treat them as text.
+# - Ensure no CRLF line-endings, neither on checkout nor on checkin.
+# - Detect whitespace errors.
+#   - Exposed by default in `git diff --color` on the CLI.
+#   - Validate with `git diff --check`.
+#   - Deny applying with `git apply --whitespace=error-all`.
+#   - Fix automatically with `git apply --whitespace=fix`.
+
+*.config  text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.css     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.dist    text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.engine  text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.html    text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=html
+*.inc     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.install text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.js      text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.json    text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.lock    text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.map     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.md      text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.module  text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.php     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.po      text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.profile text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.script  text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.sh      text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.sql     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.svg     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.theme   text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.twig    text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.txt     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.xml     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.yml     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+
+# Define binary file attributes.
+# - Do not treat them as text.
+# - Include binary diff in patches instead of "binary files differ."
+*.eot     -text diff
+*.exe     -text diff
+*.gif     -text diff
+*.gz      -text diff
+*.ico     -text diff
+*.jpeg    -text diff
+*.jpg     -text diff
+*.otf     -text diff
+*.phar    -text diff
+*.png     -text diff
+*.svgz    -text diff
+*.ttf     -text diff
+*.woff    -text diff
+*.woff2   -text diff
diff --git a/.gitignore b/.gitignore
index 3cbb8af..4a17fc5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -121,7 +121,6 @@ upstream/web/
 /web/sites/example.sites.php
 /web/themes/README.txt
 /.drush-lock-update
-/.gitattributes
 
 # Other common rules
 # ------------------
diff --git a/composer.json b/composer.json
index d20e1e6..ac57578 100644
--- a/composer.json
+++ b/composer.json
@@ -50,7 +50,8 @@
             },
             "file-mapping": {
                 "[project-root]/.editorconfig": false,
-                "[project-root]/pantheon.upstream.yml": false
+                "[project-root]/pantheon.upstream.yml": false,
+                "[project-root]/.gitattributes": false
             }
         }
     },
diff --git a/pantheon.upstream.yml b/pantheon.upstream.yml
index db4f233..5cd946a 100644
--- a/pantheon.upstream.yml
+++ b/pantheon.upstream.yml
@@ -1,5 +1,6 @@
 api_version: 1
 web_docroot: true
+# See https://pantheon.io/docs/pantheon-yml/#enforce-https--hsts for valid values.
 enforce_https: transitional
 php_version: 7.4
 drush_version: 10