diff --git a/pagetop/src/html.rs b/pagetop/src/html.rs
index 18e16fea..34cb7cdb 100644
--- a/pagetop/src/html.rs
+++ b/pagetop/src/html.rs
@@ -25,6 +25,8 @@ pub use attribute::AttributeValue;
 mod classes;
 pub use classes::{Classes, ClassesOp};
 
+pub mod unit;
+
 pub enum PrepareMarkup {
     None,
     Text(&'static str),
diff --git a/pagetop/src/html/unit.rs b/pagetop/src/html/unit.rs
new file mode 100644
index 00000000..44371fa3
--- /dev/null
+++ b/pagetop/src/html/unit.rs
@@ -0,0 +1,54 @@
+use crate::concat_string;
+
+// About pixels: Pixels (px) are relative to the viewing device. For low-dpi devices, 1px is one
+// device pixel (dot) of the display. For printers and high resolution screens 1px implies multiple
+// device pixels.
+
+// About em: 2em means 2 times the size of the current font. The em and rem units are practical in
+// creating perfectly scalable layout!
+
+// About viewport: If the browser window size is 50cm wide, 1vw = 0.5cm.
+
+#[rustfmt::skip]
+#[derive(Default)]
+pub enum Value {
+    #[default]
+    None,
+    Auto,
+
+    Cm(isize),      // Centimeters.
+    In(isize),      // Inches (1in = 96px = 2.54cm).
+    Mm(isize),      // Millimeters.
+    Pc(isize),      // Picas (1pc = 12pt).
+    Pt(isize),      // Points (1pt = 1/72 of 1in).
+    Px(isize),      // Pixels (1px = 1/96th of 1in).
+
+    RelEm(f32),     // Relative to the font-size of the element.
+    RelPct(f32),    // Percentage relative to the parent element.
+    RelRem(f32),    // Relative to font-size of the root element.
+    RelVh(f32),     // Relative to 1% of the height of the viewport.
+    RelVw(f32),     // Relative to 1% of the value of the viewport.
+}
+
+#[rustfmt::skip]
+impl ToString for Value {
+    fn to_string(&self) -> String {
+        match self {
+            Value::None       => "".to_owned(),
+            Value::Auto       => "auto".to_owned(),
+            // Absolute value.
+            Value::Cm(av)     => concat_string!(av.to_string(), "cm"),
+            Value::In(av)     => concat_string!(av.to_string(), "in"),
+            Value::Mm(av)     => concat_string!(av.to_string(), "mm"),
+            Value::Pc(av)     => concat_string!(av.to_string(), "pc"),
+            Value::Pt(av)     => concat_string!(av.to_string(), "pt"),
+            Value::Px(av)     => concat_string!(av.to_string(), "px"),
+            // Relative value.
+            Value::RelEm(rv)  => concat_string!(rv.to_string(), "em"),
+            Value::RelPct(rv) => concat_string!(rv.to_string(), "%"),
+            Value::RelRem(rv) => concat_string!(rv.to_string(), "rem"),
+            Value::RelVh(rv)  => concat_string!(rv.to_string(), "vh"),
+            Value::RelVw(rv)  => concat_string!(rv.to_string(), "vw"),
+        }
+    }
+}