diff --git a/apply-css/background.js b/apply-css/background.js
index 9397366..6528f4e 100644
--- a/apply-css/background.js
+++ b/apply-css/background.js
@@ -1,8 +1,11 @@
 const CSS = "body { border: 20px solid red; }";
-
 const TITLE_APPLY = "Apply CSS";
 const TITLE_REMOVE = "Remove CSS";
 
+/*
+Toggle CSS: based on the current title, insert or remove the CSS.
+Update the page action's title and icon to reflect its state.
+*/
 function toggleCSS(tab) {
 
   function gotTitle(title) {
@@ -20,18 +23,30 @@ function toggleCSS(tab) {
   chrome.pageAction.getTitle({tabId: tab.id}, gotTitle)
 }
 
+/*
+Initialize the page action: set icon and title, then show.
+*/
 function initializePageAction(tabId) {
   chrome.pageAction.setIcon({tabId, path: "icons/off.svg"});
   chrome.pageAction.setTitle({tabId, title: TITLE_APPLY});
   chrome.pageAction.show(tabId);
 }
 
+/*
+When first loaded, initialize the page action for all tabs.
+*/
 chrome.tabs.query({}, (tabs)=> {
   for (tab of tabs) {
     initializePageAction(tab.id);
   }
 });
 
+/*
+Each time a tab is updated, reset the page action for that tab.
+*/
 chrome.tabs.onUpdated.addListener(initializePageAction);
 
+/*
+Toggle CSS when the page action is clicked.
+*/
 chrome.pageAction.onClicked.addListener(toggleCSS);