diff --git a/README.md b/README.md
index 3e8eefb7..baba6079 100644
--- a/README.md
+++ b/README.md
@@ -97,6 +97,15 @@ The `enveloped-signature` transform removes only the `Signature` element being v
 - `getCanonXml()` finds the loaded signature in the node's document the same way, and removes nothing
   when the signature is not there.
 
+### Documents with a DOCTYPE
+
+`computeSignature()` and `checkSignature()` do not process a document type declaration. An entity
+it declares is not expanded and a default attribute it declares is not added, though
+[Canonical XML](https://www.w3.org/TR/xml-c14n#Terminology) requires both, so the canonical form is
+not the one an implementation that processes the declaration computes. Signing or verifying a
+document that has a DOCTYPE is deprecated and will throw in 7.0. Until then it prints a
+`DeprecationWarning`.
+
 ### Malformed certificates
 
 A certificate whose encapsulated data is not base64 is rejected rather than decoded as far as it
diff --git a/src/signed-xml.ts b/src/signed-xml.ts
index bbad4e61..2a7fee3b 100644
--- a/src/signed-xml.ts
+++ b/src/signed-xml.ts
@@ -108,6 +108,12 @@ const warnOriginalXmlWithIds = deprecate(
   "XML_CRYPTO_GET_ORIGINAL_XML_WITH_IDS",
 );
 
+const warnDocumentTypeDeclaration = deprecate(
+  () => {},
+  "Signing or verifying a document that has a document type declaration (`<!DOCTYPE>`) is deprecated and will throw in version 7.0. The declaration is not processed, so an entity it declares is not expanded and a default attribute it declares is not added to what is signed or verified.",
+  "XML_CRYPTO_DOCUMENT_TYPE_DECLARATION",
+);
+
 const warnValidateElementAgainstReferences = deprecate(
   () => {},
   "`validateElementAgainstReferences()` is deprecated and will be removed in a future version. After `checkSignature()` succeeds, use the XML that `getSignedReferences()` returns instead of nodes from the original document.",
@@ -365,6 +371,9 @@ export class SignedXml {
     this.signedXml = xml;
 
     const doc = new xmldom.DOMParser().parseFromString(xml);
+    if (doc.doctype != null) {
+      warnDocumentTypeDeclaration();
+    }
 
     // Reset the references as only references from our re-parsed signedInfo node can be trusted
     this.references = [];
@@ -1034,6 +1043,9 @@ export class SignedXml {
     }
 
     const doc = new xmldom.DOMParser().parseFromString(xml);
+    if (doc.doctype != null) {
+      warnDocumentTypeDeclaration();
+    }
     let xmlNsAttr = "xmlns";
     const signatureAttrs: string[] = [];
     let currentPrefix: string;