diff --git a/src/rules/relative-font-units.js b/src/rules/relative-font-units.js
index f11725f9..8d18862b 100644
--- a/src/rules/relative-font-units.js
+++ b/src/rules/relative-font-units.js
@@ -9,6 +9,7 @@
 
 /**
  * @import { CSSRuleDefinition } from "../types.js"
+ * @import { CssNode, CssNodePlain } from "@eslint/css-tree"
  * @typedef {"allowedFontUnits"} RelativeFontUnitsMessageIds
  * @typedef {[{allowUnits?: string[]}]} RelativeFontUnitsOptions
  * @typedef {CSSRuleDefinition<{ RuleOptions: RelativeFontUnitsOptions, MessageIds: RelativeFontUnitsMessageIds}>} RelativeFontUnitsRuleDefinition
@@ -46,6 +47,22 @@ const disallowedFontSizeKeywords = new Set([
 	"math",
 ]);
 
+/**
+ * Checks whether a font size uses a unit or keyword that is not allowed.
+ * @param {CssNodePlain} value The font size value.
+ * @param {string[]} allowedFontUnits The relative units that are allowed.
+ * @returns {boolean} Whether the font size value is disallowed.
+ */
+function isDisallowedFontSize(value, allowedFontUnits) {
+	return (
+		(value.type === "Dimension" &&
+			!allowedFontUnits.includes(value.unit.toLowerCase())) ||
+		(value.type === "Identifier" &&
+			disallowedFontSizeKeywords.has(value.name.toLowerCase())) ||
+		(value.type === "Percentage" && !allowedFontUnits.includes("%"))
+	);
+}
+
 //-----------------------------------------------------------------------------
 // Rule Definition
 //-----------------------------------------------------------------------------
@@ -90,6 +107,7 @@ export default /** @satisfies {RelativeFontUnitsRuleDefinition} */ ({
 
 	create(context) {
 		const [{ allowUnits: allowedFontUnits }] = context.options;
+		const { lexer } = context.sourceCode;
 
 		return {
 			Declaration(node) {
@@ -100,18 +118,7 @@ export default /** @satisfies {RelativeFontUnitsRuleDefinition} */ ({
 					) {
 						const value = node.value.children[0];
 
-						if (
-							(value.type === "Dimension" &&
-								!allowedFontUnits.includes(
-									value.unit.toLowerCase(),
-								)) ||
-							(value.type === "Identifier" &&
-								disallowedFontSizeKeywords.has(
-									value.name.toLowerCase(),
-								)) ||
-							(value.type === "Percentage" &&
-								!allowedFontUnits.includes("%"))
-						) {
+						if (isDisallowedFontSize(value, allowedFontUnits)) {
 							context.report({
 								loc: value.loc,
 								messageId: "allowedFontUnits",
@@ -130,68 +137,22 @@ export default /** @satisfies {RelativeFontUnitsRuleDefinition} */ ({
 						node.value.children.length > 0
 					) {
 						const value = node.value;
-
-						const dimensionNode = value.children.find(
-							child => child.type === "Dimension",
-						);
-						const identifierNode = value.children.find(
-							child =>
-								child.type === "Identifier" &&
-								disallowedFontSizeKeywords.has(
-									child.name.toLowerCase(),
-								),
-						);
-						const percentageNode = value.children.find(
-							(child, index) => {
-								const isPercentage =
-									child.type === "Percentage";
-								const previousNode = value.children[index - 1];
-
-								const previousNodeIsSlashOperator =
-									previousNode &&
-									previousNode.type === "Operator" &&
-									previousNode.value === "/";
-
-								return (
-									isPercentage && !previousNodeIsSlashOperator
-								);
-							},
-						);
-
-						let location;
-						let shouldReport = false;
-
-						const conditions = [
-							{
-								check:
-									!allowedFontUnits.includes("%") &&
-									percentageNode,
-								loc: percentageNode?.loc,
-							},
-							{
-								check: identifierNode,
-								loc: identifierNode?.loc,
-							},
-							{
-								check:
-									dimensionNode &&
-									!allowedFontUnits.includes(
-										dimensionNode.unit.toLowerCase(),
+						const match = lexer.matchProperty("font", value);
+						const fontSizeNode = match.matched
+							? value.children.find(child =>
+									match.isProperty(
+										/** @type {CssNode} */ (child),
+										"font-size",
 									),
-								loc: dimensionNode?.loc,
-							},
-						];
-						for (const condition of conditions) {
-							if (condition.check) {
-								shouldReport = true;
-								location = condition.loc;
-								break;
-							}
-						}
+								)
+							: undefined;
 
-						if (shouldReport) {
+						if (
+							fontSizeNode &&
+							isDisallowedFontSize(fontSizeNode, allowedFontUnits)
+						) {
 							context.report({
-								loc: location,
+								loc: fontSizeNode.loc,
 								messageId: "allowedFontUnits",
 								data: {
 									allowedFontUnits:
diff --git a/tests/rules/relative-font-units.test.js b/tests/rules/relative-font-units.test.js
index 1b69276d..549f6eaa 100644
--- a/tests/rules/relative-font-units.test.js
+++ b/tests/rules/relative-font-units.test.js
@@ -28,6 +28,8 @@ ruleTester.run("relative-font-units", rule, {
 		"a { font-size: 1rem; }",
 		"a { font: 2rem Arial, sans-serif; }",
 		"a { font: 1.2rem/2 Arial, sans-serif; }",
+		"a { font: oblique 10deg 1rem serif; }",
+		"a { font: 1rem math; }",
 		"a { font-size: 1REM; }",
 		"a { font-size: 1Rem; }",
 		"a { font-size: 1rEm; }",
@@ -764,6 +766,19 @@ ruleTester.run("relative-font-units", rule, {
 				},
 			],
 		},
+		{
+			code: "a { font: oblique 10deg 1em serif; }",
+			errors: [
+				{
+					messageId: "allowedFontUnits",
+					line: 1,
+					column: 25,
+					endLine: 1,
+					endColumn: 28,
+					data: { allowedFontUnits: "rem" },
+				},
+			],
+		},
 		{
 			code: "a { font-size: xx-small; }",
 			errors: [