diff --git a/packages/core/src/builder/functions.ts b/packages/core/src/builder/functions.ts
index 42d75e8..adcbd2b 100644
--- a/packages/core/src/builder/functions.ts
+++ b/packages/core/src/builder/functions.ts
@@ -1,7 +1,7 @@
 import { replaceIDs } from './ids';
 import { calculateSize } from './calc-size';
 import { fullIcon, IconifyIcon } from '../icon';
-import { fullCustomisations } from '../customisations';
+import { defaults, mergeCustomisations } from '../customisations';
 import type { IconifyIconCustomisations } from '../customisations';
 import { iconToSVG } from '.';
 import type { IconifyIconBuildResult } from '.';
@@ -29,6 +29,9 @@ export const builderFunctions: IconifyBuilderFunctions = {
 	replaceIDs,
 	calculateSize,
 	buildIcon: (icon, customisations) => {
-		return iconToSVG(fullIcon(icon), fullCustomisations(customisations));
+		return iconToSVG(
+			fullIcon(icon),
+			mergeCustomisations(defaults, customisations)
+		);
 	},
 };
diff --git a/packages/core/src/builder/index.ts b/packages/core/src/builder/index.ts
index d59bae1..3261fae 100644
--- a/packages/core/src/builder/index.ts
+++ b/packages/core/src/builder/index.ts
@@ -66,7 +66,7 @@ interface ViewBox {
  * Get SVG attributes and content from icon + customisations
  *
  * Does not generate style to make it compatible with frameworks that use objects for style, such as React.
- * Instead, it generates verticalAlign value that should be added to style.
+ * Instead, it generates 'inline' value. If true, rendering engine should add verticalAlign: -0.125em to icon.
  *
  * Customisations should be normalised by platform specific parser.
  * Result should be converted to <svg> by platform specific parser.
@@ -84,80 +84,102 @@ export function iconToSVG(
 		height: icon.height,
 	};
 
-	// Apply transformations
-	const transformations: string[] = [];
-	const hFlip = customisations.hFlip !== icon.hFlip;
-	const vFlip = customisations.vFlip !== icon.vFlip;
-	let rotation = customisations.rotate + icon.rotate;
+	// Body
+	let body = icon.body;
 
-	if (hFlip) {
-		if (vFlip) {
-			rotation += 2;
-		} else {
-			// Horizontal flip
+	// Apply transformations
+	[icon, customisations].forEach((props) => {
+		const transformations: string[] = [];
+		const hFlip = props.hFlip;
+		const vFlip = props.vFlip;
+		let rotation = props.rotate;
+
+		// Icon is flipped first, then rotated
+		if (hFlip) {
+			if (vFlip) {
+				rotation += 2;
+			} else {
+				// Horizontal flip
+				transformations.push(
+					'translate(' +
+						(box.width + box.left) +
+						' ' +
+						(0 - box.top) +
+						')'
+				);
+				transformations.push('scale(-1 1)');
+				box.top = box.left = 0;
+			}
+		} else if (vFlip) {
+			// Vertical flip
 			transformations.push(
 				'translate(' +
-					(box.width + box.left) +
+					(0 - box.left) +
 					' ' +
-					(0 - box.top) +
+					(box.height + box.top) +
 					')'
 			);
-			transformations.push('scale(-1 1)');
+			transformations.push('scale(1 -1)');
 			box.top = box.left = 0;
 		}
-	} else if (vFlip) {
-		// Vertical flip
-		transformations.push(
-			'translate(' + (0 - box.left) + ' ' + (box.height + box.top) + ')'
-		);
-		transformations.push('scale(1 -1)');
-		box.top = box.left = 0;
-	}
 
-	let tempValue: number;
-	rotation = rotation % 4;
-	switch (rotation) {
-		case 1:
-			// 90deg
-			tempValue = box.height / 2 + box.top;
-			transformations.unshift(
-				'rotate(90 ' + tempValue + ' ' + tempValue + ')'
-			);
-			break;
-
-		case 2:
-			// 180deg
-			transformations.unshift(
-				'rotate(180 ' +
-					(box.width / 2 + box.left) +
-					' ' +
-					(box.height / 2 + box.top) +
-					')'
-			);
-			break;
-
-		case 3:
-			// 270deg
-			tempValue = box.width / 2 + box.left;
-			transformations.unshift(
-				'rotate(-90 ' + tempValue + ' ' + tempValue + ')'
-			);
-			break;
-	}
-
-	if (rotation % 2 === 1) {
-		// Swap width/height and x/y for 90deg or 270deg rotation
-		if (box.left !== 0 || box.top !== 0) {
-			tempValue = box.left;
-			box.left = box.top;
-			box.top = tempValue;
+		let tempValue: number;
+		if (rotation < 0) {
+			rotation -= Math.floor(rotation / 4) * 4;
 		}
-		if (box.width !== box.height) {
-			tempValue = box.width;
-			box.width = box.height;
-			box.height = tempValue;
+		rotation = rotation % 4;
+		switch (rotation) {
+			case 1:
+				// 90deg
+				tempValue = box.height / 2 + box.top;
+				transformations.unshift(
+					'rotate(90 ' + tempValue + ' ' + tempValue + ')'
+				);
+				break;
+
+			case 2:
+				// 180deg
+				transformations.unshift(
+					'rotate(180 ' +
+						(box.width / 2 + box.left) +
+						' ' +
+						(box.height / 2 + box.top) +
+						')'
+				);
+				break;
+
+			case 3:
+				// 270deg
+				tempValue = box.width / 2 + box.left;
+				transformations.unshift(
+					'rotate(-90 ' + tempValue + ' ' + tempValue + ')'
+				);
+				break;
 		}
-	}
+
+		if (rotation % 2 === 1) {
+			// Swap width/height and x/y for 90deg or 270deg rotation
+			if (box.left !== 0 || box.top !== 0) {
+				tempValue = box.left;
+				box.left = box.top;
+				box.top = tempValue;
+			}
+			if (box.width !== box.height) {
+				tempValue = box.width;
+				box.width = box.height;
+				box.height = tempValue;
+			}
+		}
+
+		if (transformations.length) {
+			body =
+				'<g transform="' +
+				transformations.join(' ') +
+				'">' +
+				body +
+				'</g>';
+		}
+	});
 
 	// Calculate dimensions
 	let width, height;
@@ -195,13 +217,6 @@ export function iconToSVG(
 	width = typeof width === 'string' ? width : width + '';
 	height = typeof height === 'string' ? height : height + '';
 
-	// Generate body
-	let body = icon.body;
-	if (transformations.length) {
-		body =
-			'<g transform="' + transformations.join(' ') + '">' + body + '</g>';
-	}
-
 	// Result
 	const result: IconifyIconBuildResult = {
 		attributes: {
diff --git a/packages/core/src/customisations/index.ts b/packages/core/src/customisations/index.ts
index 8d372e4..f3fe4bc 100644
--- a/packages/core/src/customisations/index.ts
+++ b/packages/core/src/customisations/index.ts
@@ -1,5 +1,3 @@
-import { merge } from '../misc/merge';
-
 /**
  * Icon alignment
  */
@@ -57,11 +55,81 @@ export const defaults: FullIconCustomisations = Object.freeze({
 	rotate: 0,
 });
 
+/**
+ * TypeScript
+ */
+// eslint-disable-next-line @typescript-eslint/no-unused-vars-experimental, @typescript-eslint/no-unused-vars
+function assertNever(v: never) {
+	//
+}
+
 /**
  * Convert IconifyIconCustomisations to FullIconCustomisations
  */
-export function fullCustomisations(
+export function mergeCustomisations(
+	defaults: FullIconCustomisations,
 	item: IconifyIconCustomisations
 ): FullIconCustomisations {
-	return merge(defaults, item) as FullIconCustomisations;
+	const result: FullIconCustomisations = {} as FullIconCustomisations;
+	for (const key in defaults) {
+		const attr = key as keyof FullIconCustomisations;
+
+		// Copy old value
+		(result as Record<string, unknown>)[attr] = defaults[attr];
+
+		if (item[attr] === void 0) {
+			continue;
+		}
+
+		// Validate new value
+		const value = item[attr];
+		switch (attr) {
+			// Boolean attributes that override old value
+			case 'inline':
+			case 'slice':
+				if (typeof value === 'boolean') {
+					result[attr] = value;
+				}
+				break;
+
+			// Boolean attributes that are merged
+			case 'hFlip':
+			case 'vFlip':
+				if (value === true) {
+					result[attr] = !result[attr];
+				}
+				break;
+
+			// Non-empty string
+			case 'hAlign':
+			case 'vAlign':
+				if (typeof value === 'string' && value !== '') {
+					(result as Record<string, unknown>)[attr] = value;
+				}
+				break;
+
+			// Non-empty string / non-zero number / null
+			case 'width':
+			case 'height':
+				if (
+					(typeof value === 'string' && value !== '') ||
+					(typeof value === 'number' && value) ||
+					value === null
+				) {
+					result[attr] = value;
+				}
+				break;
+
+			// Rotation
+			case 'rotate':
+				if (typeof value === 'number') {
+					result[attr] += value;
+				}
+				break;
+
+			default:
+				assertNever(attr);
+		}
+	}
+	return result;
 }
diff --git a/packages/core/tests/20-builder/icon-to-svg-test.ts b/packages/core/tests/20-builder/icon-to-svg-test.ts
index 53a8d38..1e001bc 100644
--- a/packages/core/tests/20-builder/icon-to-svg-test.ts
+++ b/packages/core/tests/20-builder/icon-to-svg-test.ts
@@ -5,7 +5,7 @@ import { iconToSVG } from '../../lib/builder';
 import type { FullIconifyIcon } from '../../lib/icon';
 import { iconDefaults, fullIcon } from '../../lib/icon';
 import type { FullIconCustomisations } from '../../lib/customisations';
-import { defaults, fullCustomisations } from '../../lib/customisations';
+import { defaults, mergeCustomisations } from '../../lib/customisations';
 
 describe('Testing iconToSVG', () => {
 	it('Empty icon', () => {
@@ -26,7 +26,7 @@ describe('Testing iconToSVG', () => {
 	});
 
 	it('Auto size, inline, body', () => {
-		const custom: FullIconCustomisations = fullCustomisations({
+		const custom: FullIconCustomisations = mergeCustomisations(defaults, {
 			inline: true,
 			height: 'auto',
 		});
@@ -49,7 +49,7 @@ describe('Testing iconToSVG', () => {
 	});
 
 	it('Auto size, inline, body', () => {
-		const custom: FullIconCustomisations = fullCustomisations({
+		const custom: FullIconCustomisations = mergeCustomisations(defaults, {
 			inline: true,
 			height: 'auto',
 		});
@@ -72,7 +72,7 @@ describe('Testing iconToSVG', () => {
 	});
 
 	it('Custom size, alignment', () => {
-		const custom: FullIconCustomisations = fullCustomisations({
+		const custom: FullIconCustomisations = mergeCustomisations(defaults, {
 			height: 'auto',
 			hAlign: 'left',
 			slice: true,
@@ -97,7 +97,7 @@ describe('Testing iconToSVG', () => {
 	});
 
 	it('Rotation, alignment', () => {
-		const custom: FullIconCustomisations = fullCustomisations({
+		const custom: FullIconCustomisations = mergeCustomisations(defaults, {
 			height: '40px',
 			vAlign: 'bottom',
 			rotate: 1,
@@ -121,8 +121,32 @@ describe('Testing iconToSVG', () => {
 		expect(result).to.be.eql(expected);
 	});
 
+	it('Negative rotation', () => {
+		const custom: FullIconCustomisations = mergeCustomisations(defaults, {
+			height: '40px',
+			rotate: -1,
+		});
+		const icon: FullIconifyIcon = fullIcon({
+			width: 20,
+			height: 16,
+			body: '<path d="..." />',
+		});
+		const expected: IconifyIconBuildResult = {
+			attributes: {
+				width: '32px',
+				height: '40px',
+				preserveAspectRatio: 'xMidYMid meet',
+				viewBox: '0 0 16 20',
+			},
+			body: '<g transform="rotate(-90 10 10)"><path d="..." /></g>',
+		};
+
+		const result = iconToSVG(icon, custom);
+		expect(result).to.be.eql(expected);
+	});
+
 	it('Flip, alignment', () => {
-		const custom: FullIconCustomisations = fullCustomisations({
+		const custom: FullIconCustomisations = mergeCustomisations(defaults, {
 			height: '32',
 			vAlign: 'top',
 			hAlign: 'right',
@@ -140,8 +164,7 @@ describe('Testing iconToSVG', () => {
 				preserveAspectRatio: 'xMaxYMin meet',
 				viewBox: '0 0 20 16',
 			},
-			body:
-				'<g transform="translate(20 0) scale(-1 1)"><path d="..." /></g>',
+			body: '<g transform="translate(20 0) scale(-1 1)"><path d="..." /></g>',
 		};
 
 		const result = iconToSVG(icon, custom);
@@ -149,8 +172,8 @@ describe('Testing iconToSVG', () => {
 	});
 
 	it('Flip, rotation', () => {
-		const custom: FullIconCustomisations = fullCustomisations({
-			vFlip: true,
+		const custom: FullIconCustomisations = mergeCustomisations(defaults, {
+			hFlip: true,
 			rotate: 1,
 		});
 		const icon: FullIconifyIcon = fullIcon({
@@ -165,8 +188,34 @@ describe('Testing iconToSVG', () => {
 				preserveAspectRatio: 'xMidYMid meet',
 				viewBox: '0 0 16 20',
 			},
-			body:
-				'<g transform="rotate(90 8 8) translate(0 16) scale(1 -1)"><path d="..." /></g>',
+			body: '<g transform="rotate(90 8 8) translate(20 0) scale(-1 1)"><path d="..." /></g>',
+		};
+
+		const result = iconToSVG(icon, custom);
+		expect(result).to.be.eql(expected);
+	});
+
+	it('Flip icon that is rotated by default', () => {
+		const custom: FullIconCustomisations = mergeCustomisations(defaults, {
+			hFlip: true,
+		});
+		const icon: FullIconifyIcon = fullIcon({
+			width: 20,
+			height: 16,
+			body: '<path d="..." />',
+			rotate: 1,
+		});
+
+		// Horizontally flipping icon that has 90 or 270 degrees rotation will result in vertical flip.
+		// Therefore result should be rotation + vertical flip to visually match horizontal flip on normal icon.
+		const expected: IconifyIconBuildResult = {
+			attributes: {
+				width: '0.8em',
+				height: '1em',
+				preserveAspectRatio: 'xMidYMid meet',
+				viewBox: '0 0 16 20',
+			},
+			body: '<g transform="translate(16 0) scale(-1 1)"><g transform="rotate(90 8 8)"><path d="..." /></g></g>',
 		};
 
 		const result = iconToSVG(icon, custom);
@@ -174,7 +223,7 @@ describe('Testing iconToSVG', () => {
 	});
 
 	it('Flip and rotation canceling eachother', () => {
-		const custom: FullIconCustomisations = fullCustomisations({
+		const custom: FullIconCustomisations = mergeCustomisations(defaults, {
 			width: '1em',
 			height: 'auto',
 			hFlip: true,
@@ -204,7 +253,10 @@ describe('Testing iconToSVG', () => {
 		const iconBody =
 			'<g stroke="currentColor" stroke-width="16" stroke-linecap="round" stroke-linejoin="round" fill="none" fill-rule="evenodd"><path d="M40 64l48-48" class="animation-delay-0 animation-duration-10 animate-stroke stroke-length-102"/><path d="M40 64l48 48" class="animation-delay-0 animation-duration-10 animate-stroke stroke-length-102"/></g>';
 
-		const custom: FullIconCustomisations = fullCustomisations({});
+		const custom: FullIconCustomisations = mergeCustomisations(
+			defaults,
+			{}
+		);
 		const icon: FullIconifyIcon = fullIcon({
 			body: iconBody,
 			width: 128,