diff --git a/src/post2html b/src/post2html
index c0c6c03..51f6f5a 100755
--- a/src/post2html
+++ b/src/post2html
@@ -86,15 +86,56 @@ hgroup-wrap()
 # Pre-format Markdown files before they get to Pandoc
 #
 # These may be able to be implemented as Pandoc filters, but I haven't had
-# the time to research that yet.  This is actually a fitting real-world
-# demonstration of incremental development / MVP that I'm writing about at
-# the time that this comment was written (MyCustomBB Part I)!
+# the time to research that yet.
+#
+# I had originally switched to Markdown hoping that it would suit my needs
+# better than an ad-hoc formatting language of my own design.  And yet here
+# I am again, adding to the language.
 prefmt()
 {
+  # Accepting a filename instead of stdin allows us to output errors
+  # including the name of the file.
+  local -r file="$1"
+
   awk '
-    triml {
-      gsub( /^ +/, "" )
+    BEGIN {
+      infence = 0
+      gather = 0
+      nltrim = 0
       triml = 0
+      indent = 0
+      ex = 0
+    }
+
+    # indentation is the first non-space character
+    { indent = match($0, /[^ ]/) - 1 }
+
+    # HTML nesting
+    /^ *<\/details>/ {
+      triml = indent
+    }
+
+    triml && indent >= 0 {
+      if (indent < triml) {
+        printf "error: post2html: %s:%d: expected %d-char indent, found %d\n", \
+          FILENAME, NR, triml, indent \
+          > "/dev/stderr"
+        ex = 1
+      } else {
+        # must have `triml` leading spaces
+        $0 = substr($0, triml + 1)
+      }
+    }
+
+    # must appear _after_ triml above so we just discard what is left,
+    # or we mess up the indentation calculation
+    nltrim && indent {
+      gsub( /^ +/, "" )
+      nltrim = 0
+    }
+
+    /^ *<details\>/ {
+      triml = indent + 2
     }
 
     # ties
@@ -105,7 +146,7 @@ prefmt()
     !infence && /%$/ {
       gsub( /%$/, "" )
       printf "%s", $0
-      triml = 1
+      nltrim = 1
       next
     }
 
@@ -120,7 +161,9 @@ prefmt()
     !gather && /^ *@LASTFENCE@$/ { print gblock; next }
 
     { print }
-  '
+
+    END { exit(ex) }
+  ' "$file"
 }
 
 
@@ -139,7 +182,7 @@ main()
          --highlight-style tango \
          -B <( src/mkheader post @__PAGE_TITLE__@ ) \
          -A src/footer.tpl.htm \
-         < <( prefmt < "$file" ) \
+         < <( prefmt "$file" ) \
     | src/h12title @__PAGE_TITLE__@ \
     | hgroup-wrap "$date" "$file"
 }
diff --git a/style.css b/style.css
index 070d168..a61e220 100644
--- a/style.css
+++ b/style.css
@@ -417,10 +417,23 @@ article figure > figcaption:last-child {
   margin-top: 1em;
 }
 
+article details {
+  /* 3px border + 2px spacing for nested details */
+  border-left: 5px solid;
+  border-image: linear-gradient(to right, #eeeeee 3px, transparent 2px) 5;
+
+  padding: 0em 1em;
+  margin: 1em 0em 1em -1em;
+}
+
 article details > summary {
   cursor: pointer;
 }
 
+article details[open] > summary {
+  border-bottom: 1px dashed;
+}
+
 
 @media ( max-width: 60ch ) {
   article {