diff --git a/components/config/opts.rs b/components/config/opts.rs
index ea1173c6443..f4da0658011 100644
--- a/components/config/opts.rs
+++ b/components/config/opts.rs
@@ -889,14 +889,7 @@ pub fn from_cmdline_args(args: &[String]) -> ArgumentParsingResult {
     prefs::add_user_prefs();
 
     for pref in opt_match.opt_strs("pref").iter() {
-        let split: Vec<&str> = pref.splitn(2, '=').collect();
-        let pref_name = split[0];
-        let value = split.get(1);
-        match value {
-            Some(&"false") => PREFS.set(pref_name, PrefValue::Boolean(false)),
-            Some(&"true") | None => PREFS.set(pref_name, PrefValue::Boolean(true)),
-            _ => PREFS.set(pref_name, PrefValue::String(value.unwrap().to_string()))
-        };
+        parse_pref_from_command_line(pref);
     }
 
     if let Some(layout_threads) = layout_threads {
@@ -946,6 +939,20 @@ pub fn set_defaults(opts: Opts) {
     }
 }
 
+pub fn parse_pref_from_command_line(pref: &str) {
+    let split: Vec<&str> = pref.splitn(2, '=').collect();
+    let pref_name = split[0];
+    let value = split.get(1);
+    match value {
+        Some(&"false") => PREFS.set(pref_name, PrefValue::Boolean(false)),
+        Some(&"true") | None => PREFS.set(pref_name, PrefValue::Boolean(true)),
+        Some(value) => match value.parse::<f64>() {
+            Ok(v) => PREFS.set(pref_name, PrefValue::Number(v)),
+            Err(_) => PREFS.set(pref_name, PrefValue::String(value.to_string()))
+        }
+    };
+}
+
 #[inline]
 pub fn get() -> &'static Opts {
     &OPTIONS
diff --git a/tests/unit/servo_config/opts.rs b/tests/unit/servo_config/opts.rs
index a6827fd05e9..b533429df5b 100644
--- a/tests/unit/servo_config/opts.rs
+++ b/tests/unit/servo_config/opts.rs
@@ -2,7 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use servo_config::opts::parse_url_or_filename;
+use servo_config::opts::{parse_url_or_filename, parse_pref_from_command_line};
+use servo_config::prefs::{PrefValue, PREFS};
 use std::path::Path;
 
 #[cfg(not(target_os = "windows"))]
@@ -67,3 +68,26 @@ fn test_argument_parsing_special() {
     assert_eq!(url.query(), None);
     assert_eq!(url.fragment(), None);
 }
+
+#[test]
+fn test_parse_pref_from_command_line() {
+    // Test with boolean values.
+    parse_pref_from_command_line("testtrue=true");
+    assert_eq!(*PREFS.get("testtrue"), PrefValue::Boolean(true));
+    parse_pref_from_command_line("testfalse=false");
+    assert_eq!(*PREFS.get("testfalse"), PrefValue::Boolean(false));
+
+    // Test with numbers.
+    parse_pref_from_command_line("testint=42");
+    assert_eq!(*PREFS.get("testint"), PrefValue::Number(42 as f64));
+    parse_pref_from_command_line("testfloat=4.2");
+    assert_eq!(*PREFS.get("testfloat"), PrefValue::Number(4.2));
+
+    // Test default (string).
+    parse_pref_from_command_line("teststr=str");
+    assert_eq!(*PREFS.get("teststr"), PrefValue::String("str".to_owned()));
+
+    // Test with no value.
+    parse_pref_from_command_line("testempty");
+    assert_eq!(*PREFS.get("testempty"), PrefValue::Boolean(true));
+}