diff --git a/lib/rules/unit-no-unknown/__tests__/index.js b/lib/rules/unit-no-unknown/__tests__/index.js
index a2e8d6bec7..41319c467c 100644
--- a/lib/rules/unit-no-unknown/__tests__/index.js
+++ b/lib/rules/unit-no-unknown/__tests__/index.js
@@ -220,6 +220,22 @@ testRule(rule, {
 			code: "@import 'foo.css'",
 			description: 'ignore non-media and non-variable at-rule',
 		},
+		{
+			code: "a { background-image: image-set('img-1x.jpg' 1x, 'img-2x.jpg' 2x, 'img-3x.jpg' 3x) }",
+			description: 'ignore `x` unit in image-set',
+		},
+		{
+			code: '@media (resolution: 2x) {}',
+			description: 'ignore `x` unit in @media with `resolution`',
+		},
+		{
+			code: '@media ( resOLution:  2x) {}',
+			description: 'ignore `x` unit in @media with `resolution`',
+		},
+		{
+			code: 'a { image-resolution: 1x; }',
+			description: 'ignore `x` unit in image-resolution',
+		},
 	],
 
 	reject: [
@@ -356,6 +372,50 @@ testRule(rule, {
 			line: 1,
 			column: 13,
 		},
+		{
+			code: 'a { width: 400x; }',
+			message: messages.rejected('x'),
+			description: '`x` is not allowed for non-resolution props',
+			line: 1,
+			column: 12,
+		},
+		{
+			code: '@media (resolution: 2x) and (min-width: 200x) {}',
+			message: messages.rejected('x'),
+			description: '`x` rejected with inappropriate property',
+			line: 1,
+			column: 41,
+		},
+		{
+			code: '@media ( resolution: /* comment */ 2x ) and (min-width: 200x) {}',
+			message: messages.rejected('x'),
+			description: '`x` rejected with inappropriate property',
+			line: 1,
+			column: 44,
+		},
+		{
+			code:
+				"a { background: image-set('img1x.png' 1x, 'img2x.png' 2x) left 20x / 15% 60% repeat-x; }",
+			message: messages.rejected('x'),
+			description: '`x` rejected with inappropriate property',
+			line: 1,
+			column: 64,
+		},
+		{
+			code:
+				"a { background: /* comment */ image-set('img1x.png' 1x, 'img2x.png' 2x) left 20x / 15% 60% repeat-x; }",
+			message: messages.rejected('x'),
+			description: '`x` rejected with inappropriate property',
+			line: 1,
+			column: 78,
+		},
+		{
+			code: "a { background-image: image-set('img1x.png' 1pix, 'img2x.png' 2x); }",
+			message: messages.rejected('pix'),
+			description: '`pix` rejected with inappropriate property',
+			line: 1,
+			column: 45,
+		},
 	],
 });
 
diff --git a/lib/rules/unit-no-unknown/index.js b/lib/rules/unit-no-unknown/index.js
index 92bcfeabc8..763257bf33 100644
--- a/lib/rules/unit-no-unknown/index.js
+++ b/lib/rules/unit-no-unknown/index.js
@@ -5,6 +5,7 @@ const atRuleParamIndex = require('../../utils/atRuleParamIndex');
 const declarationValueIndex = require('../../utils/declarationValueIndex');
 const getUnitFromValueNode = require('../../utils/getUnitFromValueNode');
 const keywordSets = require('../../reference/keywordSets');
+const mediaParser = require('postcss-media-query-parser').default;
 const optionsMatches = require('../../utils/optionsMatches');
 const report = require('../../utils/report');
 const ruleMessages = require('../../utils/ruleMessages');
@@ -41,7 +42,9 @@ const rule = function(actual, options) {
 			// make sure multiplication operations (*) are divided - not handled
 			// by postcss-value-parser
 			value = value.replace(/\*/g, ',');
-			valueParser(value).walk(function(valueNode) {
+			const parsedValue = valueParser(value);
+
+			parsedValue.walk(function(valueNode) {
 				// Ignore wrong units within `url` function
 				// and within functions listed in the `ignoreFunctions` option
 				if (
@@ -62,10 +65,50 @@ const rule = function(actual, options) {
 					return;
 				}
 
-				if (keywordSets.units.has(unit.toLowerCase())) {
+				if (keywordSets.units.has(unit.toLowerCase()) && unit.toLowerCase() !== 'x') {
 					return;
 				}
 
+				if (unit.toLowerCase() === 'x') {
+					if (
+						node.type === 'atrule' &&
+						node.name === 'media' &&
+						node.params.toLowerCase().includes('resolution')
+					) {
+						let ignoreUnit = false;
+
+						mediaParser(node.params).walk((mediaNode, i, mediaNodes) => {
+							if (
+								mediaNode.value.toLowerCase().includes('resolution') &&
+								_.last(mediaNodes).sourceIndex === valueNode.sourceIndex
+							) {
+								ignoreUnit = true;
+
+								return false;
+							}
+						});
+
+						if (ignoreUnit) {
+							return;
+						}
+					}
+
+					if (node.type === 'decl') {
+						if (node.prop.toLowerCase() === 'image-resolution') {
+							return;
+						}
+
+						if (/^image-set/i.test(value)) {
+							const imageSet = parsedValue.nodes.find((node) => node.value === 'image-set');
+							const imageSetValueLastIndex = _.last(imageSet.nodes).sourceIndex;
+
+							if (imageSetValueLastIndex >= valueNode.sourceIndex) {
+								return;
+							}
+						}
+					}
+				}
+
 				report({
 					index: getIndex(node) + valueNode.sourceIndex,
 					message: messages.rejected(unit),