diff --git a/rtfd/public/html/index.html b/rtfd/public/html/index.html
index 54253c5..dff5fa7 100644
--- a/rtfd/public/html/index.html
+++ b/rtfd/public/html/index.html
@@ -7,10 +7,14 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>Title.</title>
     <link rel="stylesheet" href="style/main.css">
+    <script src="script/debug.js" defer></script>
   </head>
 
   <body>
 
+    <header id="debug">
+    </header>
+
     <header>
       <img src="images/logo.svg" alt="Logo of the website.">
       <nav aria-label="Main navigation">
diff --git a/rtfd/public/html/index.rst b/rtfd/public/html/index.rst
index d2559de..e901ce6 100644
--- a/rtfd/public/html/index.rst
+++ b/rtfd/public/html/index.rst
@@ -31,3 +31,12 @@ Main
 
 .. literalinclude:: style/main.css
    :language: css
+
+JS
+==
+
+Debug
+-----
+
+.. literalinclude:: script/debug.js
+   :language: js
diff --git a/rtfd/public/html/script/debug.js b/rtfd/public/html/script/debug.js
new file mode 100644
index 0000000..0124fa8
--- /dev/null
+++ b/rtfd/public/html/script/debug.js
@@ -0,0 +1,10 @@
+const body = document.body
+
+const style = window.getComputedStyle(body)
+
+const font_size = style.fontSize
+
+
+const debug = document.getElementById("debug")
+
+debug.innerHTML = `Font size = ${font_size}`
diff --git a/rtfd/public/html/style/main.css b/rtfd/public/html/style/main.css
index acf6090..a25bad0 100644
--- a/rtfd/public/html/style/main.css
+++ b/rtfd/public/html/style/main.css
@@ -1,4 +1,16 @@
 /*
+16 1
+17 1.0625
+18 1.125
+19 1.1875
+20 1.25
+21 1.3125
+22 1.375
+phone: 480 px
+tablet: 768 px
+laptop: 1024 px
+desktop: 1366 px
+
 block-size ← height
 (border|padding)-block[-start|end]
 (border|padding)-inline[-start|end]
@@ -21,6 +33,7 @@ background-position: center;
 background-size: cover;
 color: #777;
 font-family: monospace;
+font-size: 1rem;
 line-height: 1.25;
 margin-block: 0;
 margin-inline: auto;
