diff --git a/src/cascade.ts b/src/cascade.ts
index ed4241d..019d9e7 100644
--- a/src/cascade.ts
+++ b/src/cascade.ts
@@ -70,7 +70,7 @@ function expandInsetShorthands(node: CssNode, block?: Block) {
   };
 
   if (node.property === 'inset') {
-    const values = node.value.children.toArray();
+    const values = node.value.children?.toArray() || [];
     // `inset` shorthand expands to top, right, bottom, left
     // See https://drafts.csswg.org/css-position/#inset-shorthands
     const [top, right, bottom, left] = (() => {
@@ -92,7 +92,7 @@ function expandInsetShorthands(node: CssNode, block?: Block) {
     appendProperty('bottom', bottom);
     appendProperty('left', left);
   } else if (node.property === 'inset-block') {
-    const values = node.value.children.toArray();
+    const values = node.value.children?.toArray() || [];
     const [blockStart, blockEnd] = (() => {
       switch (values.length) {
         case 1:
@@ -106,7 +106,7 @@ function expandInsetShorthands(node: CssNode, block?: Block) {
     appendProperty('inset-block-start', blockStart);
     appendProperty('inset-block-end', blockEnd);
   } else if (node.property === 'inset-inline') {
-    const values = node.value.children.toArray();
+    const values = node.value.children?.toArray() || [];
     const [inlineStart, inlineEnd] = (() => {
       switch (values.length) {
         case 1:
diff --git a/src/fallback.ts b/src/fallback.ts
index 43ee7a4..9e88280 100644
--- a/src/fallback.ts
+++ b/src/fallback.ts
@@ -319,7 +319,7 @@ function mapMargin(
 ) {
   // TODO: Handle flip-start
   if (key === 'margin') {
-    const [first, second, third, fourth] = valueAst.children.toArray();
+    const [first, second, third, fourth] = valueAst.children?.toArray() || [];
     if (tactic === 'flip-block') {
       if (fourth) {
         valueAst.children.fromArray([third, second, first, fourth]);
@@ -332,14 +332,14 @@ function mapMargin(
       } // No change needed for 1, 2 or 3 values
     }
   } else if (key === 'margin-block') {
-    const [first, second] = valueAst.children.toArray();
+    const [first, second] = valueAst.children?.toArray() || [];
     if (tactic === 'flip-block') {
       if (second) {
         valueAst.children.fromArray([second, first]);
       }
     }
   } else if (key === 'margin-inline') {
-    const [first, second] = valueAst.children.toArray();
+    const [first, second] = valueAst.children?.toArray() || [];
     if (tactic === 'flip-inline') {
       if (second) {
         valueAst.children.fromArray([second, first]);
@@ -351,9 +351,9 @@ function mapMargin(
 // Parses a value into an AST.
 const getValueAST = (property: string, val: string) => {
   const ast = getAST(`#id{${property}: ${val};}`) as Block;
-  const astDeclaration = (ast.children.first as Rule)?.block.children
-    .first as Declaration;
-  return astDeclaration.value as Value;
+  const astDeclaration = (ast.children?.first as Rule)?.block?.children
+    ?.first as Declaration;
+  return astDeclaration?.value as Value;
 };
 
 export function applyTryTacticToBlock(
@@ -453,7 +453,7 @@ function parsePositionTryFallbacks(list: List<CssNode>) {
 }
 
 function getPositionTryFallbacksDeclaration(node: Declaration) {
-  if (isPositionTryFallbacksDeclaration(node) && node.value.children.first) {
+  if (isPositionTryFallbacksDeclaration(node) && node.value.children?.first) {
     return parsePositionTryFallbacks(node.value.children);
   }
   return [];
@@ -463,11 +463,11 @@ export function getPositionTryDeclaration(node: Declaration): {
   order?: PositionTryOrder;
   options?: PositionTryObject[];
 } {
-  if (isPositionTryDeclaration(node) && node.value.children.first) {
+  if (isPositionTryDeclaration(node) && node.value.children?.first) {
     const declarationNode = clone(node) as DeclarationWithValue;
     let order: PositionTryOrder | undefined;
     // get potential order
-    const firstName = (declarationNode.value.children.first as Identifier).name;
+    const firstName = (declarationNode.value.children?.first as Identifier)?.name;
     if (firstName && isPositionTryOrder(firstName)) {
       order = firstName;
       declarationNode.value.children.shift();
@@ -480,9 +480,9 @@ export function getPositionTryDeclaration(node: Declaration): {
 }
 
 function getPositionTryOrderDeclaration(node: Declaration) {
-  if (isPositionTryOrderDeclaration(node) && node.value.children.first) {
+  if (isPositionTryOrderDeclaration(node) && node.value.children?.first) {
     return {
-      order: (node.value.children.first as Identifier).name as PositionTryOrder,
+      order: (node.value.children.first as Identifier)?.name as PositionTryOrder,
     };
   }
   return {};
diff --git a/src/index-wpt.ts b/src/index-wpt.ts
index bb11d4d..1b7d949 100644
--- a/src/index-wpt.ts
+++ b/src/index-wpt.ts
@@ -2,13 +2,15 @@ import { polyfill } from './polyfill.js';
 
 // Used by the WPT test harness to delay test assertions
 // and give the polyfill time to apply changes
-window.CHECK_LAYOUT_DELAY = true;
+if (typeof window !== 'undefined') {
+  window.CHECK_LAYOUT_DELAY = true;
 
-// apply polyfill
-if (document.readyState !== 'complete') {
-  window.addEventListener('load', () => {
+  // apply polyfill
+  if (document.readyState !== 'complete') {
+    window.addEventListener('load', () => {
+      polyfill(true);
+    });
+  } else {
     polyfill(true);
-  });
-} else {
-  polyfill(true);
+  }
 }
diff --git a/src/index.ts b/src/index.ts
index 575e326..9b24c17 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,10 +1,12 @@
 import { polyfill } from './polyfill.js';
 
 // apply polyfill
-if (document.readyState !== 'complete') {
-  window.addEventListener('load', () => {
+if (typeof window !== 'undefined') {
+  if (document.readyState !== 'complete') {
+    window.addEventListener('load', () => {
+      polyfill();
+    });
+  } else {
     polyfill();
-  });
-} else {
-  polyfill();
+  }
 }
diff --git a/src/parse.ts b/src/parse.ts
index a91a1b5..f8112cc 100644
--- a/src/parse.ts
+++ b/src/parse.ts
@@ -142,7 +142,7 @@ function parseAnchorFn(
     customPropName: string | undefined;
 
   const args: CssNode[] = [];
-  node.children.toArray().forEach((child) => {
+  node.children?.toArray()?.forEach((child) => {
     if (foundComma) {
       fallbackValue = `${fallbackValue}${generateCSS(child)}`;
       return;
@@ -164,9 +164,9 @@ function parseAnchorFn(
     if (isIdentifier(name) && name.name.startsWith('--')) {
       // Store anchor name
       anchorName = name.name;
-    } else if (isVarFunction(name) && name.children.first) {
+    } else if (isVarFunction(name) && name.children?.first) {
       // Store CSS custom prop for anchor name
-      customPropName = (name.children.first as Identifier).name;
+      customPropName = (name.children.first as Identifier)?.name;
     }
   }
   if (sideOrSize) {
@@ -210,7 +210,7 @@ function parseAnchorFn(
 }
 
 function getAnchorNames(node: DeclarationWithValue) {
-  return (node.value.children as List<Identifier>).map(({ name }) => name);
+  return (node.value.children as List<Identifier>)?.map(({ name }) => name) || [];
 }
 
 let anchorNames: AnchorSelectors = {};
@@ -427,8 +427,8 @@ export async function parseCSS(
             isVarFunction(node) &&
             declaration &&
             prop &&
-            node.children.first &&
-            customPropsToCheck.has((node.children.first as Identifier).name) &&
+            node.children?.first &&
+            customPropsToCheck.has((node.children.first as Identifier)?.name) &&
             // For now, we only want assignments to other CSS custom properties
             prop.startsWith('--')
           ) {
@@ -496,7 +496,7 @@ export async function parseCSS(
           isVarFunction(node) &&
           declaration &&
           prop &&
-          node.children.first &&
+          node.children?.first &&
           // Now we only want assignments to inset/sizing properties
           (isInsetProp(prop) || isSizingProp(prop))
         ) {
@@ -649,7 +649,7 @@ export async function parseCSS(
         enter(node) {
           if (
             isVarFunction(node) &&
-            (node.children.first as Identifier)?.name?.startsWith('--') &&
+            (node.children?.first as Identifier)?.name?.startsWith('--') &&
             this.declaration?.property?.startsWith('--') &&
             this.block
           ) {
diff --git a/src/position-area.ts b/src/position-area.ts
index 8f3fc29..756e57f 100644
--- a/src/position-area.ts
+++ b/src/position-area.ts
@@ -457,8 +457,8 @@ function isPositionAreaDeclaration(
 
 function parsePositionAreaValue(node: DeclarationWithValue) {
   const value = (node.value.children as List<Identifier>)
-    .toArray()
-    .map(({ name }) => name);
+    ?.toArray()
+    ?.map(({ name }) => name) || [];
   if (value.length === 1) {
     if (axisForPositionAreaValue(value[0]) === 'ambiguous') {
       value.push(value[0]);
diff --git a/src/utils.ts b/src/utils.ts
index 58b7bb3..3245e24 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -61,7 +61,7 @@ export function isDeclaration(node: CssNode): node is DeclarationWithValue {
 }
 
 export function getDeclarationValue(node: DeclarationWithValue) {
-  return (node.value.children.first as Identifier).name;
+  return (node.value.children?.first as Identifier)?.name;
 }
 
 export interface StyleData {
@@ -95,7 +95,7 @@ export function getSelectors(rule: SelectorList | undefined) {
   if (!rule) return [];
 
   return (rule.children as List<CssTreeSelector>)
-    .map((selector) => {
+    ?.map((selector) => {
       let pseudoElementPart: string | undefined;
 
       if (selector.children.last?.type === 'PseudoElementSelector') {
@@ -112,7 +112,7 @@ export function getSelectors(rule: SelectorList | undefined) {
         pseudoElementPart,
       } satisfies Selector;
     })
-    .toArray();
+    .toArray() || [];
 }
 
 export function reportParseErrorsOnFailure() {