diff --git a/default_environment/stylesheet/main.css b/default_environment/stylesheet/main.css
index e69de29..387c9d9 100644
--- a/default_environment/stylesheet/main.css
+++ b/default_environment/stylesheet/main.css
@@ -0,0 +1,37 @@
+:root {
+    --main-max-width: 800px;
+}
+
+.page_centering {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+}
+
+.page_header {
+    width: 100%;
+    max-width: var(--main-max-width);
+    border-bottom: 0.2rem solid black;
+
+    .home_link {
+        font-size: 1.2rem;
+        font-weight: 600;
+        text-decoration: none;
+    }
+}
+
+.main_content {
+    width: 100%;
+    max-width: var(--main-max-width);
+    font-size: 1.2rem;
+}
+
+/* A stand-alone blog post (not in a blog stream). */
+.blog_post {
+    padding: 1rem 0 3rem 0;
+}
+
+.blog_post_title {
+    font-size: 2rem;
+    font-weight: 700;
+}
diff --git a/default_environment/templates/html/blog_post_page.dust b/default_environment/templates/html/blog_post_page.dust
index d846e3e..82cd6d7 100644
--- a/default_environment/templates/html/blog_post_page.dust
+++ b/default_environment/templates/html/blog_post_page.dust
@@ -1,8 +1,6 @@
-<div class="blog_post">
-  <div class="blog_post_intro">
-    {?.title}{?.self_link}<a class="blog_post_title" href="{.self_link}">{.title}</a>{:else}<div class="blog_post_title">{.title}</div>{/.self_link}{/.title}
-    {! TODO: date? !}
-  </div>
+<article class="blog_post">
+  {?.title}<h1>{?.self_link}<a class="blog_post_title" href="{.self_link}">{.title}</a>{:else}<span class="blog_post_title">{.title}</span>{/.self_link}</h1>{/.title}
+  {! TODO: date? !}
 
   {! TODO: Table of contents? !}
 
@@ -18,4 +16,4 @@
       {/.footnotes}
     {/.footnotes}
   </div>
-</div>
+</article>
diff --git a/default_environment/templates/html/main.dust b/default_environment/templates/html/main.dust
index 9f0f5ed..d183145 100644
--- a/default_environment/templates/html/main.dust
+++ b/default_environment/templates/html/main.dust
@@ -6,14 +6,14 @@
     {#global_settings.js_files}<script type="text/javascript" src="{.}"></script>{/global_settings.js_files}
     {?global_settings.page_title}<title>{global_settings.page_title}</title>{/global_settings.page_title}
   </head>
-  <body>
+  <body class="page_centering">
     {#.page_header}{>page_header/}{/.page_header}
-    <div class="main_content">
+    <main class="main_content">
       {@select key=.type}
         {@eq value="blog_post_page"}{>blog_post_page/}{/eq}
         {@eq value="blog_stream"}{>blog_stream/}{/eq}
         {@none}{!TODO: make this panic!}ERROR: Unrecognized page content type{/none}
       {/select}
-    </div>
+    </main>
   </body>
 </html>
diff --git a/default_environment/templates/html/page_header.dust b/default_environment/templates/html/page_header.dust
index f2c3089..d14af84 100644
--- a/default_environment/templates/html/page_header.dust
+++ b/default_environment/templates/html/page_header.dust
@@ -1,4 +1,4 @@
-<div class="page_header">
-  <a href="{.home_link}">{.website_title}</a>
-  {! TODO: Additional links? !}
-</div>
+<header class="page_header">
+  <a class="home_link" href="{.home_link}">{.website_title}</a>
+  {! TODO: Additional links? Probably using the nav semantic element. !}
+</header>
