diff --git a/_plugins/pa11y.rb b/_plugins/pa11y.rb
new file mode 100644
index 000000000..1027dd188
--- /dev/null
+++ b/_plugins/pa11y.rb
@@ -0,0 +1,66 @@
+=begin
+
+Determines which files to check with pa11y-ci per pull request.
+
+The current scope of pa11y testing is as follows:
+
+- Check any posts or pages whose content has changed.
+- Check any posts or pages using changed layouts.
+- If stylesheets have changed, check a sample of all page types:
+    - Three posts of each layout in _config.yml's `defaults`
+    - The first, last, and middle blog archive pages (/blog/page/:num)
+
+Previously, checking every page resulted in excessive CI runtimes.
+
+=end
+
+# Wraps Jekyll documents with convenience methods.
+#
+# @param path [String] The relative path of the source file, usually a
+#    Markdown file (.md)
+# @param layout [String] The name of the Jekyll document's layout
+# @param differ [Object, #changed_files] The collaborator which lists
+#   the files that were changed in the last commit.
+# @todo Are there cases when the document being checked has no layout?
+Document = Struct.new(:path, :layout, :differ) do
+  # @return [Boolean] Whether to scan this document in pa11y-ci
+  def to_scan?
+    source_changed? || layout_changed?
+  end
+
+  def source_changed?
+    differ.changed_files.include?(path)
+  end
+
+  def layout_changed?
+    differ.changed_files.include?("_layouts/#{layout}.html")
+  end
+end
+
+# Gets the list of files changed in the last commit
+# @note Designed as a collaborator object to make Document easier
+#   to test
+module GitDiffer
+  # @return [Array<String>] List of relative paths of files that were
+  #   changed in the last commit.
+  def self.changed_files
+    @changed_files ||= %x[ git diff-tree --no-commit-id --name-only -r $(git rev-parse HEAD) ]
+      .to_s
+      .split("\n")
+      .map(&:strip)
+  end
+end
+
+# Outputs posts and pages to scan in file `pa11y_targets`.
+# @todo Implement the stylesheet checker (third bullet above)
+Jekyll::Hooks.register :documents, :post_render do |doc|
+  puts doc.relative_path if doc.relative_path.match?(/css/)
+  document = Document.new(
+    doc.relative_path,
+    doc.data["layout"],
+    GitDiffer
+  )
+  if document.to_scan?
+    File.open('pa11y_targets', 'a') { |f| f.write doc.destination("") }
+  end
+end
diff --git a/spec/_plugins/pa11y_spec.rb b/spec/_plugins/pa11y_spec.rb
new file mode 100644
index 000000000..d77499637
--- /dev/null
+++ b/spec/_plugins/pa11y_spec.rb
@@ -0,0 +1,68 @@
+require_relative '../../_plugins/pa11y'
+
+RSpec.describe Document do
+
+  let(:blog_path) { "_posts/2024-04-10-working-with-oracle-databases.md" }
+  let(:blog_layout) { "post" }
+  let(:blog_layout_path) { "_layouts/post.html" }
+
+  # Stand-in for file diffing (GitDiffer), so tests don't rely on
+  # git history.
+  class TestDiffer
+    attr_reader :changed_files
+
+    def initialize(*paths)
+      @changed_files = *paths
+    end
+  end
+
+  # @example Use the convenience method
+  #   TestDiffer("_layouts/default.html", "_posts/2024-04-10-new-post.md")
+  def TestDiffer(*paths)
+    TestDiffer.new(*paths)
+  end
+
+  describe "#to_scan?" do
+    context "source files" do
+      let(:document) {
+        Document.new(blog_path, blog_layout, test_differ)
+      }
+
+      context "with a matching changed source file" do
+        let(:test_differ) { TestDiffer(blog_path) }
+        it "returns true" do
+          expect(document.to_scan?).to be true
+        end
+      end
+
+      context "without a matching changed source file" do
+        let(:test_differ) { TestDiffer() }
+        it "returns false" do
+          expect(document.to_scan?).to be false
+        end
+      end
+
+      context "with a matching changed layout file" do
+        let(:test_differ) { TestDiffer(blog_layout_path) }
+        it "returns true" do
+          expect(document.to_scan?).to be true
+        end
+      end
+
+      context "without a matching changed layout file" do
+        let(:test_differ) { TestDiffer() }
+        it "returns false" do
+          expect(document.to_scan?).to be false
+        end
+      end
+
+      context "with a matching changed source file and changed layout file" do
+        let(:test_differ) { TestDiffer(blog_path, blog_layout_path) }
+        it "returns true" do
+          expect(document.to_scan?).to be true
+        end
+      end
+    end
+  end
+
+end