diff --git a/crates/treehouse/src/cli/generate.rs b/crates/treehouse/src/cli/generate.rs
index fb7b52c..ac75fa5 100644
--- a/crates/treehouse/src/cli/generate.rs
+++ b/crates/treehouse/src/cli/generate.rs
@@ -24,7 +24,7 @@ use crate::{
         tree::branches_to_html,
     },
     state::Source,
-    tree::SemaRoots,
+    tree::{attributes::RootAttributes, SemaRoots},
 };
 
 use crate::state::{FileId, Treehouse};
diff --git a/crates/treehouse/src/html/tree.rs b/crates/treehouse/src/html/tree.rs
index 4037f3e..bbb50d6 100644
--- a/crates/treehouse/src/html/tree.rs
+++ b/crates/treehouse/src/html/tree.rs
@@ -56,9 +56,9 @@ pub fn branch_to_html(
                 BranchKind::Expanded => "<details open>",
                 BranchKind::Collapsed => "<details>",
             });
-            s.push_str("<summary>");
+            s.push_str("<summary class=\"branch-container\">");
         } else {
-            s.push_str("<div>");
+            s.push_str("<div class=\"branch-container\">");
         }
 
         s.push_str("<th-bp></th-bp>");
diff --git a/crates/treehouse/src/tree/attributes.rs b/crates/treehouse/src/tree/attributes.rs
index d8e30c3..ce88408 100644
--- a/crates/treehouse/src/tree/attributes.rs
+++ b/crates/treehouse/src/tree/attributes.rs
@@ -1,13 +1,21 @@
-use serde::Deserialize;
+use serde::{Deserialize, Serialize};
 
 /// Top-level `%%` root attributes.
-#[derive(Debug, Default, Clone, PartialEq, Eq, Deserialize)]
+#[derive(Debug, Default, Clone, PartialEq, Eq, Deserialize, Serialize)]
 pub struct RootAttributes {
     /// Title of the generated .html page.
     ///
     /// The page's tree path is used if empty.
     #[serde(default)]
     pub title: String,
+
+    /// Summary of the generated .html page.
+    #[serde(default)]
+    pub description: Option<String>,
+
+    /// ID of picture attached to the page, to be used as a thumbnail.
+    #[serde(default)]
+    pub thumbnail: Option<String>,
 }
 
 /// Branch attributes.
diff --git a/static/css/main.css b/static/css/main.css
index baef92b..48e3a58 100644
--- a/static/css/main.css
+++ b/static/css/main.css
@@ -29,7 +29,9 @@
         --link-color-visited: #f7afde;
 
         --shaded-background: #f7e5df0c;
+        --border-0: #f7e5df1a;
         --border-1: #f7e5df26;
+        --border-1-and-a-half: #f7e5df3a;
         --border-2: #f7e5df4d;
     }
 }
diff --git a/static/css/tree.css b/static/css/tree.css
index aeeadc3..02a656c 100644
--- a/static/css/tree.css
+++ b/static/css/tree.css
@@ -22,6 +22,13 @@
     }
 }
 
+/*** Variables ***/
+
+:root {
+    --tree-indent-width: 28px;
+    --transition-duration: 0.15s;
+}
+
 /*** Breadcrumbs ***/
 
 .breadcrumbs {
@@ -69,14 +76,36 @@
 
 /*** Tree ***/
 
-/* Compute an indent level appropriate for the viewport. */
-.tree ul {
-    padding-left: clamp(8px, 2vw, 24px);
+.tree {
+    --tree-indent-guide-dim: transparent;
+    --tree-indent-guide-highlighted: transparent;
 }
 
-/* Top level should not have an indent. */
+.tree:has(.branch-container:hover) {
+    --tree-indent-guide-dim: transparent;
+    --tree-indent-guide-highlighted: var(--border-1);
+}
+
+/* Use an indent level appropriate for the viewport. */
+.tree ul {
+    --tree-responsive-indent-width: clamp(8px, 2vw, var(--tree-indent-width));
+
+    /* Draw indent guides. */
+    padding-left: calc(var(--tree-responsive-indent-width) / 2);
+    margin-left: calc(var(--tree-responsive-indent-width) / 2);
+    border-left: 1px solid var(--tree-indent-guide-dim);
+
+    transition: border-left-color var(--transition-duration);
+}
+
+.tree details:has(.branch-container:hover)>ul {
+    border-left-color: var(--tree-indent-guide-highlighted);
+}
+
+/* Top level should not have an indent or a border. */
 .tree>ul {
     padding-left: 0;
+    border-left: none;
 }
 
 .tree details {
@@ -107,9 +136,12 @@
     &>div {
         box-sizing: border-box;
 
+        margin-bottom: -1px;
+        border-bottom: 1px solid transparent;
+        transition: border-bottom-color var(--transition-duration);
+
         &:hover {
-            border-bottom: 1px solid var(--border-1);
-            margin-bottom: -1px;
+            border-bottom-color: var(--border-1);
         }
     }
 
@@ -130,6 +162,7 @@
 
     /* Give it a shaded background on hover */
     background-color: transparent;
+    transition: background-color var(--transition-duration);
 
     &:hover {
         background-color: var(--shaded-background);
@@ -155,7 +188,7 @@
 /* bp - bullet point */
 th-bp {
     display: block;
-    width: 28px;
+    width: var(--tree-indent-width);
     height: 24px;
 
     background-image: var(--icon-leaf);
@@ -219,6 +252,7 @@ th-bb {
 
     /* Keep the button bar invisible by default. */
     opacity: 0%;
+    transition: opacity var(--transition-duration);
 }
 
 /* When the parent is hovered over, display the button bar. */
@@ -280,10 +314,24 @@ th-bb .branch-date {
 
     &>details>summary,
     &>div {
-        border-bottom: 1px dashed var(--border-2);
-        margin-bottom: -1px;
-        border-bottom-left-radius: 0;
-        border-bottom-right-radius: 0;
+        background-color: var(--shaded-background);
+        background: linear-gradient(to right,
+                transparent 12.5%, var(--shaded-background),
+                transparent, var(--shaded-background),
+                transparent, var(--shaded-background) 87.5%);
+        background-size: 800% 100%;
+        background-position-x: 100%;
+        animation: hey-listen 2s linear;
+    }
+}
+
+@keyframes hey-listen {
+    0% {
+        background-position-x: 0%;
+    }
+
+    100% {
+        background-position-x: 100%;
     }
 }