diff --git a/lib/github-pages/configuration.rb b/lib/github-pages/configuration.rb
index 3b1932d7..f8ce274b 100644
--- a/lib/github-pages/configuration.rb
+++ b/lib/github-pages/configuration.rb
@@ -22,6 +22,8 @@ class Configuration
         "input" => "GFM",
         "hard_wrap" => false,
         "gfm_quirks" => "paragraph_end",
+        "math_engine" => "mathjax",
+        "syntax_highlighter" => "rouge",
         "syntax_highlighter_opts" => {
           "default_lang" => "plaintext",
         },
@@ -54,8 +56,6 @@ class Configuration
       "highlighter" => "rouge",
       "kramdown" => {
         "template" => "",
-        "math_engine" => "mathjax",
-        "syntax_highlighter" => "rouge",
       },
       "gist" => {
         "noscript" => false,
@@ -141,12 +141,20 @@ def set!(site)
       # Ensure we're using Kramdown or GFM.  Force to Kramdown if
       # neither of these.
       #
+      # Also override non-nil values for kramdown options math_engine and
+      # syntax_highlighter.
+      #
       # This can get called multiply on the same config, so try to
       # be idempotentish.
       def restrict_and_config_markdown_processor(config)
         config["markdown"] = "kramdown" unless \
           %w(kramdown gfm commonmarkghpages).include?(config["markdown"].to_s.downcase)
 
+        %w(math_engine syntax_highlighter).each do |opt|
+          config["kramdown"][opt] = \
+            config["kramdown"][opt] == "nil" ? nil : DEFAULTS["kramdown"][opt]
+        end
+
         return unless config["markdown"].to_s.casecmp("gfm").zero?
 
         config["markdown"] = "CommonMarkGhPages"
diff --git a/spec/github-pages/configuration_spec.rb b/spec/github-pages/configuration_spec.rb
index c13e2dc7..df99e7f1 100644
--- a/spec/github-pages/configuration_spec.rb
+++ b/spec/github-pages/configuration_spec.rb
@@ -49,6 +49,25 @@
       expect(effective_config["quiet"]).to eql(true)
     end
 
+    it "has default values for math_engine and syntax_highlighter" do
+      expect(effective_config["kramdown"]["math_engine"]).to eql("mathjax")
+      expect(effective_config["kramdown"]["syntax_highlighter"]).to eql("rouge")
+    end
+
+    it "respects 'nil' for math_engine and syntax_highlighter" do
+      config = configuration.merge("kramdown" => { "math_engine" => "nil", "syntax_highlighter" => "nil" })
+      effective_config = described_class.effective_config(config)
+      expect(effective_config["kramdown"]["math_engine"]).to be_nil
+      expect(effective_config["kramdown"]["syntax_highlighter"]).to be_nil
+    end
+
+    it "overrides non-'nil' for math_engine and syntax_highlighter" do
+      config = configuration.merge("kramdown" => { "math_engine" => "foo", "syntax_highlighter" => "bar" })
+      effective_config = described_class.effective_config(config)
+      expect(effective_config["kramdown"]["math_engine"]).to eql("mathjax")
+      expect(effective_config["kramdown"]["syntax_highlighter"]).to eql("rouge")
+    end
+
     it "passes passthroughs" do
       expect(effective_config["quiet"]).to eql(true)
       expect(effective_config["source"]).to eql(fixture_dir)