diff --git a/crates/hdx_ast/src/css/properties/mod.rs b/crates/hdx_ast/src/css/properties/mod.rs
index 5a66b6e..9adb8c9 100644
--- a/crates/hdx_ast/src/css/properties/mod.rs
+++ b/crates/hdx_ast/src/css/properties/mod.rs
@@ -22,7 +22,7 @@ impl<'a> WriteCss<'a> for Custom<'a> {
 
 impl<'a> Parse<'a> for Custom<'a> {
 	fn parse(parser: &mut Parser<'a>) -> ParserResult<Self> {
-		let old_state = parser.set_state(State::StopOnSemicolon);
+		let old_state = parser.set_state(State::StopOnSemicolon | State::Nested);
 		parser
 			.parse::<ComponentValues>()
 			.inspect_err(|_| {
diff --git a/crates/hdx_ast/src/css/stylerule.rs b/crates/hdx_ast/src/css/stylerule.rs
index 77d35b5..8542575 100644
--- a/crates/hdx_ast/src/css/stylerule.rs
+++ b/crates/hdx_ast/src/css/stylerule.rs
@@ -1,4 +1,7 @@
-use crate::css::{properties::Property, selector::SelectorList};
+use crate::{
+	css::{properties::Property, selector::SelectorList},
+	syntax::BadDeclaration,
+};
 use hdx_derive::Visitable;
 use hdx_parser::{Block, Parse, Parser, QualifiedRule, Result as ParserResult, Spanned, Vec};
 use hdx_writer::{CssWriter, OutputOption, Result as WriterResult, WriteCss};
@@ -24,6 +27,7 @@ impl<'a> Parse<'a> for StyleRule<'a> {
 impl<'a> QualifiedRule<'a> for StyleRule<'a> {
 	type Block = StyleDeclaration<'a>;
 	type Prelude = SelectorList<'a>;
+	type BadDeclaration = BadDeclaration;
 }
 
 impl<'a> WriteCss<'a> for StyleRule<'a> {
@@ -114,6 +118,7 @@ mod tests {
 		assert_parse!(StyleRule, ":nth-child(1) {\n\topacity: 0;\n}");
 		assert_parse!(StyleRule, ".foo {\n\t--bar: (baz);\n}");
 		assert_parse!(StyleRule, ".foo {\n\twidth: calc(1px + (var(--foo)) + 1px);\n}");
+		assert_parse!(StyleRule, ".foo {--bar:1}", ".foo {\n\t--bar: 1;\n}");
 	}
 
 	#[test]
diff --git a/crates/hdx_ast/src/syntax.rs b/crates/hdx_ast/src/syntax.rs
index c3aa59d..5962f1a 100644
--- a/crates/hdx_ast/src/syntax.rs
+++ b/crates/hdx_ast/src/syntax.rs
@@ -384,9 +384,48 @@ impl<'a> Parse<'a> for QualifiedRule<'a> {
 	}
 }
 
+pub struct BadDeclaration;
+// https://drafts.csswg.org/css-syntax-3/#consume-the-remnants-of-a-bad-declaration
+impl<'a> Parse<'a> for BadDeclaration {
+	fn parse(parser: &mut Parser<'a>) -> ParserResult<Self> {
+		// To consume the remnants of a bad declaration from a token stream input, given a bool nested:
+		//
+		// Process input:
+		loop {
+			let token = parser.peek::<Token![Any]>().unwrap();
+			dbg!(parser.at_end(), token);
+			//
+			// <eof-token>
+			// <semicolon-token>
+			//
+			//     Discard a token from input, and return nothing.
+			if parser.at_end() || token.kind() == Kind::Semicolon {
+				parser.hop(token);
+				return Ok(Self);
+			}
+			// <}-token>
+			//
+			//     If nested is true, return nothing. Otherwise, discard a token.
+			if token.kind() == Kind::RightCurly {
+				if parser.is(State::Nested) {
+					return Ok(Self);
+				} else {
+					parser.hop(token);
+				}
+			}
+			// anything else
+			//
+			//     Consume a component value from input, and do nothing.
+			//
+			parser.parse::<ComponentValue>()?;
+		}
+	}
+}
+
 impl<'a> QualifiedRuleTrait<'a> for QualifiedRule<'a> {
 	type Block = Block<'a>;
 	type Prelude = ComponentValues<'a>;
+	type BadDeclaration = BadDeclaration;
 }
 
 impl<'a> WriteCss<'a> for QualifiedRule<'a> {
diff --git a/crates/hdx_lexer/src/lib.rs b/crates/hdx_lexer/src/lib.rs
index ad4eecd..f483add 100644
--- a/crates/hdx_lexer/src/lib.rs
+++ b/crates/hdx_lexer/src/lib.rs
@@ -32,6 +32,12 @@ pub enum Feature {
 #[derive(Copy, Clone, PartialEq, Default, Hash)]
 pub struct LexerCheckpoint(pub(crate) Token);
 
+impl LexerCheckpoint {
+	pub fn span(&self) -> Span {
+		self.0.span()
+	}
+}
+
 pub struct Lexer<'a> {
 	source: &'a str,
 	current_token: Token,
diff --git a/crates/hdx_parser/src/cursor.rs b/crates/hdx_parser/src/cursor.rs
index 2c344fb..f1823d6 100644
--- a/crates/hdx_parser/src/cursor.rs
+++ b/crates/hdx_parser/src/cursor.rs
@@ -1,4 +1,4 @@
-use hdx_lexer::{Include, Kind, LexerCheckpoint, Token};
+use hdx_lexer::{Include, Kind, LexerCheckpoint, Span, Token};
 
 use crate::Parser;
 
@@ -8,6 +8,12 @@ pub struct ParserCheckpoint {
 	errors_pos: u8,
 }
 
+impl ParserCheckpoint {
+	pub fn span(&self) -> Span {
+		self.checkpoint.span()
+	}
+}
+
 impl<'a> Parser<'a> {
 	#[inline]
 	pub fn offset(&self) -> u32 {
diff --git a/crates/hdx_parser/src/traits/mod.rs b/crates/hdx_parser/src/traits/mod.rs
index a4b44b0..81fa782 100644
--- a/crates/hdx_parser/src/traits/mod.rs
+++ b/crates/hdx_parser/src/traits/mod.rs
@@ -105,7 +105,9 @@ pub trait StyleSheet<'a>: Sized + Parse<'a> {
 				return Ok(rules);
 			}
 			discard!(parser, CdcOrCdo);
-			rules.push(Self::Rule::parse_spanned(parser)?);
+			if let Ok(rule) = Self::Rule::parse_spanned(parser) {
+				rules.push(rule)
+			}
 		}
 	}
 }
diff --git a/crates/hdx_parser/src/traits/rules.rs b/crates/hdx_parser/src/traits/rules.rs
index d04517a..3a96ea3 100644
--- a/crates/hdx_parser/src/traits/rules.rs
+++ b/crates/hdx_parser/src/traits/rules.rs
@@ -1,5 +1,5 @@
 use hdx_atom::Atom;
-use hdx_lexer::{Kind, Spanned};
+use hdx_lexer::{Kind, Span, Spanned};
 use smallvec::{smallvec, SmallVec};
 
 use crate::{diagnostics, discard, parser::Parser, Delim, Result, State, Token, Vec};
@@ -62,18 +62,31 @@ pub trait QualifiedRule<'a>: Sized + Parse<'a> {
 	type Prelude: Parse<'a>;
 	type Block: Parse<'a>;
 
+	// QualifiedRules must be able to consume a bad declaration, for when
+	// a custom property like declaration is encountered.
+	type BadDeclaration: Parse<'a>;
+
 	// QualifiedRules must have a prelude, consequently parse_prelude must be
 	// implemented.
 	// parse_prelude is called right away, so could start with any token.
 	fn parse_prelude(parser: &mut Parser<'a>) -> Result<Spanned<Self::Prelude>> {
-		Self::Prelude::parse_spanned(parser)
+		parser.parse_spanned::<Self::Prelude>()
 	}
 
 	// QualifiedRules must have a block, consequently parse_prelude must be
 	// implemented.
 	// parse_block will always start with a {-token.
 	fn parse_block(parser: &mut Parser<'a>) -> Result<Spanned<Self::Block>> {
-		Self::Block::parse_spanned(parser)
+		parser.parse_spanned::<Self::Block>()
+	}
+
+	// QualifiedRules must be able to consume a block from their input when encountering
+	// a custom property like declaration that doesn't end but opens a `{` block. This
+	// is implemented as parsing the existing block as that' simplifies downstream logic
+	// but consumers of this trait can instead opt to implement an optimised version of
+	// this which doesn't build up an AST and just throws away tokens.
+	fn consume_block(parser: &mut Parser<'a>) {
+		parser.parse::<Self::Block>().ok();
 	}
 
 	// https://drafts.csswg.org/css-syntax-3/#consume-a-qualified-rule
@@ -96,32 +109,38 @@ pub trait QualifiedRule<'a>: Sized + Parse<'a> {
 			}
 		}
 
-		let mut potential_custom = false;
-
 		// <{-token>
-		//	If the first two non-<whitespace-token> values of rule’s prelude are an <ident-token> whose value starts with "--" followed by a <colon-token>....
+		//	If the first two non-<whitespace-token> values of rule’s prelude are an <ident-token> whose value starts with "--" followed by a <colon-token>, then:
 		let checkpoint = parser.checkpoint();
 		if let Some(token) = parser.peek::<Token![Ident]>() {
 			if token.is_dashed_ident() {
 				parser.hop(token);
-				potential_custom = parser.peek::<Delim![:]>().is_some();
+				if parser.peek::<Delim![:]>().is_some() {
+					// If nested is true, consume the remnants of a bad declaration from input, with nested set to true, and return nothing.
+					if parser.is(State::Nested) {
+						parser.rewind(checkpoint);
+						parser.parse::<Self::BadDeclaration>()?;
+						let token = parser.peek::<Token![Any]>().unwrap();
+						Err(diagnostics::BadDeclaration(Span {
+							start: checkpoint.span().start,
+							end: token.span().end,
+						}))?;
+					// If nested is false, consume a block from input, and return nothing.
+					} else {
+						Self::consume_block(parser);
+						let token = parser.peek::<Token![Any]>().unwrap();
+						Err(diagnostics::BadDeclaration(Span {
+							start: checkpoint.span().start,
+							end: token.span().end,
+						}))?;
+					}
+				}
 				parser.rewind(checkpoint);
 			}
 		}
 
-		let prelude = Self::parse_prelude(parser);
+		let mut prelude = Self::parse_prelude(parser);
 
-		// <{-token>
-		//	If the first two non-<whitespace-token> values of rule’s prelude are an <ident-token> whose value starts with "--" followed by a <colon-token>, then:
-		if potential_custom && parser.is(State::Nested) {
-			// If nested is true, consume the remnants of a bad declaration from input, with nested set to true, and return nothing.
-			parser.rewind(checkpoint);
-			todo!();
-		} else if potential_custom {
-			// If nested is false, consume a block from input, and return nothing.
-			parser.rewind(checkpoint);
-			todo!();
-		}
 		// Otherwise, consume a block from input, and let child rules be the result.
 		// If the first item of child rules is a list of declarations,
 		// remove it from child rules and assign it to rule’s declarations.