Square bracket notation for custom values in Tailwind - Mini Project: Build & Apply

Using Square Bracket Notation for Custom Values in Tailwind CSS

📖 Scenario: You are building a simple webpage section that needs a custom background color and custom padding size not available in the default Tailwind CSS palette.

🎯 Goal: Create a <div> element styled with Tailwind CSS using square bracket notation to apply a custom background color #4a90e2 and custom padding of 2.5rem.

📋 What You'll Learn

Create a <div> element with Tailwind CSS classes.

Use square bracket notation to set the background color to #4a90e2.

Use square bracket notation to set the padding to 2.5rem.

Add text inside the <div> that says 'Custom Styled Box'.

Ensure the code is valid HTML5 and uses Tailwind CSS classes correctly.

💡 Why This Matters

🌍 Real World

Web designers often need to use colors or sizes that are not included in the default Tailwind CSS palette. Square bracket notation lets them add these custom values easily without changing the Tailwind config.

💼 Career

Knowing how to use square bracket notation in Tailwind CSS helps frontend developers quickly apply custom styles, making their designs more flexible and unique while keeping code clean and maintainable.

1

Create the HTML skeleton with a <div>

Write the basic HTML structure with a <div> element containing the text Custom Styled Box inside the <body>.

Tailwind

    
    
    Custom Tailwind Box
    


    

Hint

Use a <div> tag inside the <body> and write the text exactly as shown.

2

Add a custom background color using square bracket notation

Add a Tailwind CSS class to the <div> that sets the background color to #4a90e2 using square bracket notation. Use the class bg-[#4a90e2].

Tailwind

    
    
    Custom Tailwind Box
    


    
    Custom Styled Box

Hint

Add the class bg-[#4a90e2] inside the class attribute of the <div>.

3

Add custom padding using square bracket notation

Add a Tailwind CSS class to the <div> that sets padding to 2.5rem using square bracket notation. Use the class p-[2.5rem] alongside the existing background color class.

Tailwind

    
    
    Custom Tailwind Box
    


    
    Custom Styled Box

Hint

Add the class p-[2.5rem] next to bg-[#4a90e2] inside the class attribute.

4

Add text color for better contrast

Add a Tailwind CSS class to the <div> that sets the text color to white using the class text-white, along with the existing classes.

Tailwind

    
    
    Custom Tailwind Box
    


    
    Custom Styled Box

Hint

Add the class text-white next to the other classes inside the class attribute.

Practice

1. What does the square bracket notation in Tailwind CSS allow you to do?

easy

A. Disable Tailwind's default styles

B. Write JavaScript code inside Tailwind classes

C. Create new Tailwind utility classes permanently

D. Use custom CSS values not included in Tailwind's default set

Solution

1. Step 1: Understand Tailwind's default values

   Tailwind provides a set of predefined values for spacing, colors, sizes, etc.

2. Step 2: Role of square bracket notation

   Square bracket notation lets you write any CSS value directly inside a class, like mt-[10px] for margin-top 10 pixels.

3. Final Answer:

   Use custom CSS values not included in Tailwind's default set -> Option D

4. Quick Check:

   Square brackets = custom CSS values [OK]

Hint: Square brackets mean custom CSS values inside Tailwind classes [OK]

Common Mistakes:

• Thinking it creates new permanent classes
• Confusing with JavaScript code inside classes
• Assuming it disables Tailwind styles

2. Which of the following is the correct Tailwind class to set a width of exactly 250 pixels using square bracket notation?

easy

A. width-[250]

B. w-[250px]

C. w-250px

D. w-[250]

Solution

1. Step 1: Identify correct Tailwind prefix for width

   Tailwind uses w- prefix for width utilities.

2. Step 2: Use square brackets with full CSS value

   Inside brackets, you must include the unit, so 250px is correct.

3. Final Answer:

   w-[250px] -> Option B

4. Quick Check:

   Width with unit inside brackets = w-[250px] [OK]

Hint: Use full CSS value with units inside brackets after utility prefix [OK]

Common Mistakes:

• Omitting units inside brackets
• Using wrong prefix like width-
• Not using brackets for custom values

3. What will be the rendered width of a div with class class="w-[15rem]" in Tailwind CSS?

medium

A. 15 rem units (usually 15 times root font size)

B. 15 pixels

C. 15 percent width

D. 15 viewport width units

Solution

1. Step 1: Understand the CSS unit inside brackets

   The class w-[15rem] sets width to 15 rem units, where 1 rem equals the root font size (usually 16px).

2. Step 2: Interpret the rendered size

   15 rem means 15 times the root font size, so it is not pixels or percent but relative to font size.

3. Final Answer:

   15 rem units (usually 15 times root font size) -> Option A

4. Quick Check:

   Square bracket value with rem = relative font size width [OK]

Hint: Read units inside brackets literally as CSS units [OK]

Common Mistakes:

• Assuming rem means pixels
• Confusing rem with percent or viewport units
• Ignoring units inside brackets

4. You wrote class="p-[20]" to add padding of 20 pixels, but it doesn't work. What is the error?

medium

A. Padding cannot be customized in Tailwind

B. Square brackets are not allowed for padding

C. Missing unit inside square brackets

D. Class name should be padding-[20px]

Solution

1. Step 1: Check the syntax of the custom value

   Inside square brackets, you must include the CSS unit, like 20px, not just a number.

2. Step 2: Understand Tailwind's requirement for units

   Without units, the CSS value is invalid, so the style won't apply.

3. Final Answer:

   Missing unit inside square brackets -> Option C

4. Quick Check:

   Always include units inside brackets for custom values [OK]

Hint: Always add units like px, rem inside brackets for spacing [OK]

Common Mistakes:

• Omitting units inside brackets
• Using wrong class names like padding-
• Thinking padding can't be customized

5. You want a button with a background color exactly #1a73e8 and a border radius of 12px using Tailwind's square bracket notation. Which class combination is correct?

hard

A. bg-[#1a73e8] rounded-[12px]

B. bg-#1a73e8 rounded-12px

C. background-[#1a73e8] border-radius-[12px]

D. bg-[#1a73e8] rounded-[12]

Solution

1. Step 1: Use correct Tailwind prefixes

   Background color uses bg- and border radius uses rounded-.

2. Step 2: Apply square bracket notation with full CSS values

   Inside brackets, include the full hex color #1a73e8 and the unit 12px for radius.

3. Step 3: Check for syntax correctness

   bg-[#1a73e8] rounded-[12px] uses correct prefixes and proper bracket notation with units and hash symbol.

4. Final Answer:

   bg-[#1a73e8] rounded-[12px] -> Option A

5. Quick Check:

   Use correct prefixes + full CSS values in brackets [OK]

Hint: Use bg- and rounded- with full CSS values inside brackets [OK]

Common Mistakes:

• Omitting # in hex color
• Missing units for border radius
• Using wrong prefixes like background-