diff --git a/package-lock.json b/package-lock.json
index 5c522b8..e0e6e26 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -12,7 +12,6 @@
         "async": "^3.2.1",
         "diff-match-patch": "^1.0.5",
         "kleur": "^4.1.4",
-        "parse5-sax-parser": "^6.0.1",
         "remark-parse": "^10.0.1",
         "supports-color": "^9.1.0",
         "unified": "^10.1.1"
@@ -1804,19 +1803,6 @@
         "url": "https://github.com/sponsors/wooorm"
       }
     },
-    "node_modules/parse5": {
-      "version": "6.0.1",
-      "resolved": "https://registry.npmjs.org/parse5/-/parse5-6.0.1.tgz",
-      "integrity": "sha512-Ofn/CTFzRGTTxwpNEs9PP93gXShHcTq255nzRYSKe8AkVpZY7e1fpmTfOyoIvjP5HG7Z2ZM7VS9PPhQGW2pOpw=="
-    },
-    "node_modules/parse5-sax-parser": {
-      "version": "6.0.1",
-      "resolved": "https://registry.npmjs.org/parse5-sax-parser/-/parse5-sax-parser-6.0.1.tgz",
-      "integrity": "sha512-kXX+5S81lgESA0LsDuGjAlBybImAChYRMT+/uKCEXFBFOeEhS52qUCydGhU3qLRD8D9DVjaUo821WK7DM4iCeg==",
-      "dependencies": {
-        "parse5": "^6.0.1"
-      }
-    },
     "node_modules/path-exists": {
       "version": "4.0.0",
       "resolved": "https://registry.npmjs.org/path-exists/-/path-exists-4.0.0.tgz",
@@ -3594,19 +3580,6 @@
         "is-hexadecimal": "^2.0.0"
       }
     },
-    "parse5": {
-      "version": "6.0.1",
-      "resolved": "https://registry.npmjs.org/parse5/-/parse5-6.0.1.tgz",
-      "integrity": "sha512-Ofn/CTFzRGTTxwpNEs9PP93gXShHcTq255nzRYSKe8AkVpZY7e1fpmTfOyoIvjP5HG7Z2ZM7VS9PPhQGW2pOpw=="
-    },
-    "parse5-sax-parser": {
-      "version": "6.0.1",
-      "resolved": "https://registry.npmjs.org/parse5-sax-parser/-/parse5-sax-parser-6.0.1.tgz",
-      "integrity": "sha512-kXX+5S81lgESA0LsDuGjAlBybImAChYRMT+/uKCEXFBFOeEhS52qUCydGhU3qLRD8D9DVjaUo821WK7DM4iCeg==",
-      "requires": {
-        "parse5": "^6.0.1"
-      }
-    },
     "path-exists": {
       "version": "4.0.0",
       "resolved": "https://registry.npmjs.org/path-exists/-/path-exists-4.0.0.tgz",
diff --git a/package.json b/package.json
index 2cb5c76..15a4ab5 100644
--- a/package.json
+++ b/package.json
@@ -40,7 +40,6 @@
     "async": "^3.2.1",
     "diff-match-patch": "^1.0.5",
     "kleur": "^4.1.4",
-    "parse5-sax-parser": "^6.0.1",
     "remark-parse": "^10.0.1",
     "supports-color": "^9.1.0",
     "unified": "^10.1.1"
diff --git a/src/main.js b/src/main.js
index 6e7b6ca..481c3cf 100644
--- a/src/main.js
+++ b/src/main.js
@@ -5,7 +5,6 @@ import remarkParse from 'remark-parse'
 import async from 'async'
 import supportsColor from 'supports-color'
 import color from 'kleur'
-import saxParser from 'parse5-sax-parser'
 import { exec } from 'child_process'
 import DMP from 'diff-match-patch'
 
@@ -338,15 +337,99 @@ const runTests = (queue, options) => {
   }
 }
 
-const extractHtmlComments = function(input){
-  var comments, x$, p;
-  comments = [];
-  x$ = p = new saxParser();
-  x$.on('comment', function(it){
-    return comments.push(it.text);
-  });
-  x$.end(input);
-  return comments;
+const extractHtmlComments = function(input, nodePositionInMarkdown){
+  // Reference: https://html.spec.whatwg.org/#comments
+  //
+  // Comments are generally `<!-- stuff -->`, where `stuff` is disallowed from
+  // containing the ending delimiter.  However, the comment delimiters may also
+  // occur inside CDATA blocks, where we do *not* want to parse them.
+
+  const comments = []
+
+  const CDATA_OPENER = '<![CDATA['
+  const CDATA_CLOSER = ']]>'
+  const COMMENT_OPENER = '<!--'
+  const COMMENT_CLOSER = '-->'
+  const IN_CDATA = Symbol('parser in CDATA')
+  const IN_COMMENT = Symbol('parser in comment')
+  const BASE = Symbol('parser in base state')
+  let state = BASE
+  let nextIndex = 0
+  let done = false
+
+  while (!done) {
+    const rest = input.slice(nextIndex)
+
+    switch (state) {
+      case BASE:
+        // Parse the rest of whichever we see first.  CDATA "swallows"
+        // comments, and vice-versa.
+        const cdataIndex = rest.indexOf(CDATA_OPENER)
+        const commentIndex = rest.indexOf(COMMENT_OPENER)
+
+        if (cdataIndex === -1 && commentIndex === -1) { // No more of either; done
+          done = true
+        } else if (cdataIndex === -1 && commentIndex >= 0) { // Comment only
+          state = IN_COMMENT
+          nextIndex += commentIndex
+        } else if (cdataIndex >= 0 && commentIndex === -1) { // CDATA only
+          state = IN_CDATA
+          nextIndex += cdataIndex
+        } else { // Matched both.  Go with the earlier one.
+          if (cdataIndex < commentIndex) { // CDATA earlier
+            state = IN_CDATA
+            nextIndex += cdataIndex
+          } else { // Comment earlier
+            state = IN_COMMENT
+            nextIndex += commentIndex
+          }
+        }
+        break
+
+      case IN_COMMENT: {
+        // Parse end of comment
+        const closerIndex = rest.indexOf(COMMENT_CLOSER)
+        if (closerIndex >= 0) {
+          comments.push(rest.slice(0, closerIndex))
+          nextIndex += closerIndex
+          state = BASE
+        } else {
+          // Unterminated comment
+          const openerIndex = input.slice(nextIndex)
+          const line = input.slice(0, nextIndex).split('\n').length
+            + nodePositionInMarkdown.start.line - 1
+          parsingError(`'<!--'`, 'unterminated HTML comment', {
+            location: formatPosition({ start: { line }, end: { line } }),
+            'how to fix': `Terminate the comment with '-->' where appropriate.`
+              + `\nCheck that '-->' doesn't occur anywhere unexpected.`
+          })
+        }
+        break
+      }
+
+      case IN_CDATA: {
+        // Parse end of CDATA
+        const closerIndex = rest.indexOf(CDATA_CLOSER)
+        if (closerIndex >= 0) {
+          nextIndex += closerIndex
+          state = BASE
+        } else {
+          // Unterminated CDATA
+          const line = input.slice(0, nextIndex).split('\n').length
+            + nodePositionInMarkdown.start.line - 1
+          parsingError(`'<![CDATA['`, 'unterminated HTML CDATA section', {
+            location: formatPosition({ start: { line }, end: { line } }),
+            'how to fix': `Terminate the CDATA section with ']]>'`
+              + ` where appropriate.`
+              + `\nCheck that ']]>' doesn't occur anywhere unexpected.`
+          })
+        }
+        break
+      }
+    }
+  }
+
+  return comments
 };
 
 /*
@@ -462,7 +545,7 @@ const parseAndRunTests = (text, options={jobs: 1}) => {
   const visitMarkdownNode = (node) => {
 
     if (node.type === 'html') {
-      extractHtmlComments(node.value).forEach((comment) => {
+      extractHtmlComments(node.value, node.position).forEach((comment) => {
 
         // Optional whitespace, followed by '!test', more optional whitespace,
         // then the commands we actually care about.
diff --git a/test.ls b/test.ls
index c1c773f..f0df4c9 100644
--- a/test.ls
+++ b/test.ls
@@ -258,6 +258,66 @@ txm-expect do
 
   """
 
+txm-expect do
+  name: "using << heredocs in comments works"
+  input: """
+  <!-- !test program node <<X
+  console.log('YEAH')
+  X -->
+
+  <!-- !test in test name -->
+
+      doesn't matter
+
+  <!-- !test out test name -->
+
+      YEAH
+
+  """
+  expect-stdout: """
+  TAP version 13
+  1..1
+  ok 1 test name
+
+  # 1/1 passed
+  # OK
+
+  """
+
+txm-expect do
+  name: "comment inside CDATA is not parsed"
+  input: """
+  <![CDATA[
+    This would be an error if it were parsed:
+    <!-- !test in whatever -->
+  ]]>
+  """
+  expect-stdout: /1..0\n# no tests/
+
+txm-expect do
+  name: "unterminated comment"
+  expect-exit: 2
+  input: """
+  <!-- !test check whatever
+
+    true
+
+  """
+  expect-stdout: /unterminated HTML comment[\s\S]*line 1/
+
+txm-expect do
+  name: "unterminated CDATA section"
+  expect-exit: 2
+  input: """
+  other stuff
+  <![CDATA[
+  <!-- !test check whatever -->
+
+      true
+
+  """
+  expect-stdout: /unterminated HTML CDATA[\s\S]*line 2/
+
 txm-expect do
   name: "no program specified"
   input: """