diff --git a/src/lib/util/text.test.ts b/src/lib/util/text.test.ts
index 88967da7..0ca95fd1 100644
--- a/src/lib/util/text.test.ts
+++ b/src/lib/util/text.test.ts
@@ -4,6 +4,8 @@ import {
   calculateHeaderSize,
   calculateInlineTitleSize,
   isHeader,
+  isPx,
+  pxToRem,
 } from './text';
 
 describe('calculateFontTextSize', () => {
@@ -62,3 +64,33 @@ describe('calculateHeaderSize', () => {
     expect(calculateHeaderSize('header-1')).toBe(32);
   });
 });
+
+describe('pxToRem', () => {
+  it('should convert px to rem', () => {
+    expect(pxToRem(16)).toBe(1);
+    expect(pxToRem(24)).toBe(1.5);
+    expect(pxToRem(32)).toBe(2);
+  });
+
+  it('should convert px to rem with different base value', () => {
+    expect(pxToRem(16, 20)).toBe(0.8);
+    expect(pxToRem(24, 20)).toBe(1.2);
+    expect(pxToRem(32, 20)).toBe(1.6);
+  });
+});
+
+describe('isPx', () => {
+  it('should return `true` when value is in px format', () => {
+    expect(isPx('16px')).toBe(true);
+    expect(isPx('0px')).toBe(true);
+    expect(isPx('5903px')).toBe(true);
+  });
+
+  it('should return `false` when value is in not px format', () => {
+    expect(isPx('px')).toBe(false);
+    expect(isPx('')).toBe(false);
+    expect(isPx('16rem')).toBe(false);
+    expect(isPx('16em')).toBe(false);
+    expect(isPx('16ch')).toBe(false);
+  });
+});
diff --git a/src/lib/util/text.ts b/src/lib/util/text.ts
index a86c554c..0b331205 100644
--- a/src/lib/util/text.ts
+++ b/src/lib/util/text.ts
@@ -58,7 +58,7 @@ const calculateHeaderSize = (header: HTMLHeader | HeaderToken): number => {
   }
 
   // If there is some `calc` operation going on, it has to be evaluated.
-  if (headerComputedStyle.contains('calc')) {
+  if (headerComputedStyle.includes('calc')) {
     const temp = document.createElement('div');
 
     temp.style.setProperty('font-size', `var(--${htmlHeader}-size)`);