Square bracket notation for custom values in Tailwind - Browser Rendering Trace

Render Flow - Square bracket notation for custom values

[Read Tailwind class] -> [Detect square bracket notation] -> [Parse custom value inside brackets] -> [Generate CSS rule with custom value] -> [Apply CSS to element] -> [Render updated style]

The browser reads the Tailwind class with square brackets, extracts the custom value inside, creates a CSS rule using that value, and applies it to the element for rendering.

Render Steps - 3 Steps

Code Added:class="bg-[#3490dc]"

Before

[__________]
|          |
|          |
|          |
|__________|

→

After

[##########]
|          |
|          |
|          |
|__________|

The background color changes to a custom blue (#3490dc) using square bracket notation.

🔧 Browser Action:Parses custom background color and applies CSS background-color property.

Code Sample

A box with a custom blue background color, padding of 2 rem, and text size of 1.5 rem using Tailwind's square bracket notation.

Tailwind

<div class="bg-[#3490dc] p-[2rem] text-[1.5rem]">
  Custom styled box
</div>

Render Quiz - 3 Questions

After applying step 1, what visual change do you see?

A Padding increases inside the box

B Text size becomes larger

C Background color changes to a custom blue

D No visible change

Common Confusions - 2 Topics

Why doesn't my custom value work without square brackets?

Tailwind only recognizes custom values inside square brackets. Without brackets, it looks for predefined classes and ignores unknown values.

💡 Always wrap custom values in square brackets to apply them.

Can I use any CSS value inside the brackets?

You can use most valid CSS values, but some complex values might need escaping or won't work as expected.

💡 Use simple, valid CSS values inside brackets for best results.

Property Reference

Property	Square Bracket Syntax	Value Type	Visual Effect	Common Use
Background Color	bg-[#hexcode]	Color code	Changes background color	Custom brand colors
Padding	p-[value]	Length (rem, px, etc.)	Adds space inside element edges	Custom spacing
Text Size	text-[value]	Length (rem, px)	Changes font size	Custom typography sizes

Concept Snapshot

Tailwind's square bracket notation lets you use custom CSS values. Syntax: property-[value], e.g., bg-[#3490dc], p-[2rem], text-[1.5rem]. It generates CSS with your exact value. Always wrap custom values in brackets. Great for colors, spacing, font sizes beyond defaults.

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-