diff --git a/build.sh b/build.sh
index 063f9f05..384ec269 100755
--- a/build.sh
+++ b/build.sh
@@ -654,14 +654,7 @@ function processSource {
   $QUIET || echo "Pre-processing the source..."
   cp -p  entities/out/entities.inc "$HTML_CACHE"
   cp -p  entities/out/entities-dtd.url "$HTML_CACHE"
-  if hash html-build 2>/dev/null; then
-    html-build <"$HTML_SOURCE/$source_location" >"$HTML_TEMP/source-whatwg-complete"
-  else
-    local cargo_args=( --release )
-    $VERBOSE && cargo_args+=( --verbose )
-    $QUIET && cargo_args+=( --quiet )
-    cargo run "${cargo_args[@]}" <"$HTML_SOURCE/$source_location" >"$HTML_TEMP/source-whatwg-complete"
-  fi
+  runRustTools <"$HTML_SOURCE/$source_location" >"$HTML_TEMP/source-whatwg-complete"
 
   runWattsi "$HTML_TEMP/source-whatwg-complete" "$HTML_TEMP/wattsi-output"
   if [[ $WATTSI_RESULT == "0" ]]; then
@@ -690,7 +683,7 @@ function processSource {
 
   if [[ $build_type == "default" ]]; then
     # Singlepage HTML
-    mv "$HTML_TEMP/wattsi-output/index-html" "$HTML_OUTPUT/index.html"
+    runRustTools --singlepage-post <"$HTML_TEMP/wattsi-output/index-html" >"$HTML_OUTPUT/index.html"
 
     if [[ $SINGLE_PAGE_ONLY == "false" ]]; then
       # Singlepage Commit Snapshot
@@ -750,6 +743,22 @@ function checkWattsi {
   fi
 }
 
+# Runs the Rust-based build tools, either with the version in $PATH or by using cargo to compile
+# them beforehand.
+# - Arguments: all arguments to pass to the tools
+# - Output: whatever the tools output
+function runRustTools {
+  if hash html-build 2>/dev/null; then
+    html-build "$@"
+  else
+    local cargo_args=( --release )
+    $VERBOSE && cargo_args+=( --verbose )
+    $QUIET && cargo_args+=( --quiet )
+    cargo_args+=( -- )
+    cargo run "${cargo_args[@]}" "$@"
+  fi
+}
+
 # Runs Wattsi on the given file, either locally or using the web service
 # - Arguments:
 #   - $1: the file to run Wattsi on
diff --git a/src/anchor_permanence.rs b/src/anchor_permanence.rs
new file mode 100644
index 00000000..a8a1c144
--- /dev/null
+++ b/src/anchor_permanence.rs
@@ -0,0 +1,160 @@
+//! Postprocess step for ensuring anchor permanence: see
+//! https://whatwg.org/working-mode#anchors.
+//!
+//! Scans for the `<script type="text/required-ids">` element, which lists
+//! (whitespace-separated) IDs that must appear somewhere in the document.
+//! After verifying that all listed IDs are present, removes the script element.
+
+use crate::dom_utils::NodeHandleExt;
+use html5ever::{QualName, local_name, ns};
+use markup5ever_rcdom::Handle;
+use std::collections::HashSet;
+
+pub struct Processor {
+    required_ids: HashSet<String>,
+    script_node: Option<Handle>,
+}
+
+impl Processor {
+    pub fn new() -> Self {
+        Self {
+            required_ids: HashSet::new(),
+            script_node: None,
+        }
+    }
+
+    pub fn visit(&mut self, node: &Handle) {
+        // Capture and parse the <script type="text/required-ids"> element exactly once.
+        if node.is_html_element(&local_name!("script")) {
+            const TYPE: QualName = QualName {
+                prefix: None,
+                ns: ns!(),
+                local: local_name!("type"),
+            };
+            if node.get_attribute(&TYPE).as_deref() == Some("text/required-ids") {
+                assert!(
+                    self.script_node.is_none(),
+                    "multiple required-ids scripts encountered"
+                );
+                self.script_node = Some(node.clone());
+                // Gather all text within the script and split on any ASCII whitespace.
+                let content = node.text_content();
+                for id_token in content.split_ascii_whitespace() {
+                    if !id_token.is_empty() {
+                        self.required_ids.insert(id_token.to_string());
+                    }
+                }
+            }
+        }
+
+        // For elements with an id attribute, mark the ID as seen.
+        if self.required_ids.is_empty() {
+            return;
+        }
+        const ID_QN: QualName = QualName {
+            prefix: None,
+            ns: ns!(),
+            local: local_name!("id"),
+        };
+        if let Some(id) = node.get_attribute(&ID_QN) {
+            self.required_ids.remove(id.as_ref());
+        }
+    }
+
+    pub fn apply(self) -> std::io::Result<()> {
+        if !self.required_ids.is_empty() {
+            let mut missing: Vec<_> = self.required_ids.into_iter().collect();
+            missing.sort();
+            return Err(std::io::Error::new(
+                std::io::ErrorKind::InvalidData,
+                format!(
+                    "Missing required IDs for anchor permanence: {}",
+                    missing.join(", ")
+                ),
+            ));
+        }
+
+        // Remove the script element (if present) after verification.
+        if let Some(script) = self.script_node {
+            script.remove();
+        }
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::dom_utils;
+    use crate::parser::{parse_document_async, tests::serialize_for_test};
+
+    #[tokio::test]
+    async fn removes_script_from_head() {
+        let document = parse_document_async(r#"<!DOCTYPE html>
+<html><head><script type="text/required-ids">a b c</script></head><body><div id="a"></div><p id="b"></p><section id="c"></section></body></html>
+"#.as_bytes()).await.unwrap();
+        let mut processor = Processor::new();
+        dom_utils::scan_dom(&document, &mut |h| processor.visit(h));
+        processor.apply().unwrap();
+        let serialized = serialize_for_test(&[document]);
+        assert!(!serialized.contains("text/required-ids"));
+    }
+
+    #[tokio::test]
+    async fn no_script_present_noop() {
+        let document = parse_document_async(
+            r#"<!DOCTYPE html>
+<html><head></head><body></body></html>
+"#
+            .as_bytes(),
+        )
+        .await
+        .unwrap();
+        let before = serialize_for_test(&[document.clone()]);
+        let mut processor = Processor::new();
+        dom_utils::scan_dom(&document, &mut |h| processor.visit(h));
+        processor.apply().unwrap();
+        assert_eq!(before, serialize_for_test(&[document]));
+    }
+
+    #[tokio::test]
+    async fn whitespace_splitting() {
+        // Includes indentation, multiple spaces, and newlines in the script content.
+        let document = parse_document_async(r#"<!DOCTYPE html><html><head><script type="text/required-ids">
+        foo   bar
+            baz
+    qux
+</script></head><body><div id="foo"></div><div id="bar"></div><div id="baz"></div><div id="qux"></div></body></html>
+"#.as_bytes()).await.unwrap();
+        let mut processor = Processor::new();
+        dom_utils::scan_dom(&document, &mut |h| processor.visit(h));
+        processor.apply().unwrap();
+        let serialized = serialize_for_test(&[document]);
+        assert!(!serialized.contains("text/required-ids"));
+    }
+
+    #[tokio::test]
+    async fn errors_on_missing_ids() {
+        let document = parse_document_async(r#"<!DOCTYPE html>
+<html><head><script type="text/required-ids">foo bar baz</script></head><body><div id="foo"></div></body></html>
+"#.as_bytes()).await.unwrap();
+        let mut processor = Processor::new();
+        dom_utils::scan_dom(&document, &mut |h| processor.visit(h));
+        let err = processor.apply().expect_err("expected missing IDs error");
+        assert!(
+            err.to_string()
+                .contains("Missing required IDs for anchor permanence: bar, baz")
+        );
+    }
+
+    #[tokio::test]
+    #[should_panic(expected = "multiple required-ids scripts encountered")]
+    async fn panics_on_multiple_required_ids_scripts() {
+        let document = parse_document_async(r#"<!DOCTYPE html><html><head>
+<script type="text/required-ids">a b</script>
+<script type="text/required-ids">c d</script>
+</head><body><div id="a"></div><div id="b"></div><div id="c"></div><div id="d"></div></body></html>"#.as_bytes()).await.unwrap();
+        let mut processor = Processor::new();
+        dom_utils::scan_dom(&document, &mut |h| processor.visit(h));
+    }
+}
diff --git a/src/dom_utils.rs b/src/dom_utils.rs
index 6a788e8d..dffb9fcb 100644
--- a/src/dom_utils.rs
+++ b/src/dom_utils.rs
@@ -76,6 +76,11 @@ pub trait NodeHandleExt {
     where
         Self: Sized;
 
+    /// Removes the node from its parent.
+    fn remove(&self)
+    where
+        Self: Sized;
+
     /// Clones the node and its entire subtree (including template contents).
     fn deep_clone(&self) -> Self;
 
@@ -326,6 +331,10 @@ impl NodeHandleExt for Handle {
         self.parent.take();
     }
 
+    fn remove(&self) {
+        self.replace_with(Vec::new());
+    }
+
     fn deep_clone(&self) -> Handle {
         use NodeData::*;
         let new_node_data = match &self.data {
diff --git a/src/main.rs b/src/main.rs
index 853403ed..a3ad9085 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -8,6 +8,7 @@ use std::path::{Path, PathBuf};
 
 use markup5ever_rcdom::SerializableHandle;
 
+mod anchor_permanence;
 mod annotate_attributes;
 mod boilerplate;
 mod dom_utils;
@@ -21,15 +22,25 @@ mod tag_omission;
 
 #[tokio::main]
 async fn main() -> io::Result<()> {
+    let is_post = env::args().any(|a| a == "--singlepage-post");
+    let result = if is_post {
+        // --singlepage-post runs the postprocess phase, which is currently only meant to be used on the
+        // singlepage output from Wattsi.
+        run_postprocess().await
+    } else {
+        // By default we run the preprocess phase, which creates a new input for Wattsi.
+        run_preprocess().await
+    };
+
     // This gives slightly prettier error-printing.
-    if let Err(e) = run().await {
+    if let Err(e) = result {
         eprintln!("{e}");
         std::process::exit(1);
     }
     Ok(())
 }
 
-async fn run() -> io::Result<()> {
+async fn run_preprocess() -> io::Result<()> {
     // Since we're using Rc in the DOM implementation, we must ensure that tasks
     // which act on it are confined to this thread.
 
@@ -79,6 +90,27 @@ async fn run() -> io::Result<()> {
     Ok(())
 }
 
+// The steps and considerations here are similar to run_preprocess.
+async fn run_postprocess() -> io::Result<()> {
+    let document = parser::parse_document_async(tokio::io::stdin()).await?;
+
+    let mut anchor_permanence = anchor_permanence::Processor::new();
+
+    dom_utils::scan_dom(&document, &mut |h| {
+        anchor_permanence.visit(h);
+    });
+
+    anchor_permanence.apply()?;
+
+    let serializable: SerializableHandle = document.into();
+    serialize(
+        &mut BufWriter::with_capacity(128 * 1024, io::stdout()),
+        &serializable,
+        SerializeOpts::default(),
+    )?;
+    Ok(())
+}
+
 fn path_from_env<'a, V, D>(var: &V, default: &'a D) -> Cow<'a, Path>
 where
     V: AsRef<OsStr> + ?Sized,