Content blocks with @content in SASS - Performance & Optimization

Performance: Content blocks with @content

LOW IMPACT

This affects CSS preprocessing time and the final CSS bundle size, indirectly influencing page load speed.

Reusing styles with dynamic content blocks in Sass

SASS

@mixin button-style($color) {
  background-color: $color;
  border-radius: 0.5rem;
  padding: 1rem 2rem;
  font-weight: bold;
  @content;
}

.button-primary {
  @include button-style(blue) {
    color: white;
    text-transform: uppercase;
  }
}

.button-secondary {
  @include button-style(gray) {
    color: white;
    text-transform: uppercase;
  }
}

Using @content allows injecting unique styles without repeating common styles, reducing CSS duplication.

📈 Performance Gainreduces CSS bundle size by avoiding repeated rules

Reusing styles with dynamic content blocks in Sass

SASS

@mixin button-style($color) {
  background-color: $color;
  border-radius: 0.5rem;
  padding: 1rem 2rem;
  font-weight: bold;
  // Repeating styles without @content
  color: white;
  text-transform: uppercase;
}

.button-primary {
  @include button-style(blue);
  color: white;
  text-transform: uppercase;
}

.button-secondary {
  @include button-style(gray);
  color: white;
  text-transform: uppercase;
}

Repeating the same styles inside each mixin call causes duplicated CSS rules, increasing bundle size.

📉 Performance Costadds unnecessary CSS bytes, increasing bundle size and load time

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Repeating styles without @content	No impact	No impact	No impact	[X] Bad - larger CSS bundle
Using @content to inject styles	No impact	No impact	No impact	[OK] Good - smaller CSS bundle

Rendering Pipeline

Sass @content is processed at build time, so it does not affect browser rendering directly but impacts CSS size and load time.

→CSS Preprocessing

→Network Load

→Style Calculation

⚠️ BottleneckNetwork Load due to larger CSS files

Optimization Tips

1Use @content to inject unique styles inside reusable mixins.

2Avoid repeating common styles in multiple mixin calls to reduce CSS size.

3Remember @content is processed at build time, so it does not affect runtime rendering.

Performance Quiz - 3 Questions

How does using @content in Sass mixins affect CSS bundle size?

A It increases CSS bundle size by duplicating styles.

B It helps reduce CSS bundle size by avoiding repeated styles.

C It has no effect on CSS bundle size.

D It slows down browser rendering directly.

DevTools: Network

How to check: Open DevTools, go to Network tab, filter by CSS files, and compare file sizes before and after using @content.

What to look for: Smaller CSS file size indicates better performance due to less duplicated CSS.

Practice

1. What is the main purpose of @content in Sass?

easy

A. To pass custom styles into a mixin for flexible reuse

B. To define variables inside a mixin

C. To import external CSS files

D. To create loops in Sass

Solution

1. Step 1: Understand what @content does

   @content allows you to insert custom CSS blocks inside a mixin, making it flexible.

2. Step 2: Compare with other options

   Variables, imports, and loops are different Sass features unrelated to @content.

3. Final Answer:

   To pass custom styles into a mixin for flexible reuse -> Option A

4. Quick Check:

   @content = custom styles in mixin [OK]

Hint: Remember: @content inserts custom styles inside mixins [OK]

Common Mistakes:

• Confusing @content with variables or imports
• Thinking @content creates loops
• Assuming @content is for defining selectors

2. Which of the following is the correct syntax to use @content inside a mixin?

easy

A. @mixin example { @include content; }

B. @mixin example { content(); }

C. @mixin example { @content; }

D. @mixin example { content; }

Solution

1. Step 1: Recall the syntax for content blocks

   Inside a mixin, @content; is used to insert the passed block.

2. Step 2: Check each option

   Only @mixin example { @content; } uses @content; correctly with the semicolon.

3. Final Answer:

   @mixin example { @content; } -> Option C

4. Quick Check:

   Correct syntax uses @content; inside mixin [OK]

Hint: Use @content; with a semicolon inside mixins [OK]

Common Mistakes:

• Omitting the @ symbol before content
• Using parentheses like a function call
• Forgetting the semicolon after @content

3. What will be the output CSS of this Sass code?

@mixin card {
  border: 1px solid black;
  padding: 1rem;
  @content;
}

.box {
  @include card {
    background-color: yellow;
  }
}

medium

A. .box { border: 1px solid black; padding: 1rem; } .box { background-color: yellow; }

B. .box { border: 1px solid black; padding: 1rem; background-color: yellow; }

C. .box { background-color: yellow; }

D. Syntax error, no output

Solution

1. Step 1: Understand mixin with @content

   The mixin card adds border and padding, then inserts the passed block with @content.

2. Step 2: Check how .box includes the mixin

   The block inside @include card { ... } adds background-color: yellow; inside the mixin's content.

3. Final Answer:

   .box { border: 1px solid black; padding: 1rem; background-color: yellow; } -> Option B

4. Quick Check:

   Mixin + @content block = combined styles [OK]

Hint: Mixin styles + @content block styles combine inside selector [OK]

Common Mistakes:

• Thinking @content styles appear outside the selector
• Expecting separate CSS blocks for mixin and content
• Assuming syntax error due to block usage

4. Identify the error in this Sass code using @content:

@mixin highlight {
  background: yellow;
  @content
}

.text {
  @include highlight {
    color: red;
  }
}

medium

A. Missing semicolon after @content

B. Mixin name should be @content

C. Cannot use blocks with @include

D. Background property is invalid

Solution

1. Step 1: Check syntax of @content inside mixin

   @content must end with a semicolon to be valid.

2. Step 2: Verify other parts

   Mixin name and block usage are correct; background property is valid CSS.

3. Final Answer:

   Missing semicolon after @content -> Option A

4. Quick Check:

   @content requires semicolon [OK]

Hint: Always put semicolon after @content inside mixins [OK]

Common Mistakes:

• Omitting semicolon after @content
• Confusing mixin and content names
• Thinking block usage with @include is invalid

5. You want to create a reusable button style mixin that allows custom hover styles using @content. Which Sass code correctly achieves this?

hard

A. @mixin button { padding: 1rem; border: none; @content; &:hover { @content; } }

B. @mixin button { padding: 1rem; border: none; @content; } .btn:hover { @content; }

C. @mixin button { padding: 1rem; border: none; } .btn { @include button; &:hover { @content; } }

D. @mixin button { padding: 1rem; border: none; @content; &:hover { @content; } } .btn { @include button { color: blue; &:hover { color: red; } } }

Solution

1. Step 1: Understand the goal

   Create a mixin that sets base button styles and allows custom styles including hover via @content.

2. Step 2: Analyze each option

   @mixin button { padding: 1rem; border: none; @content; &:hover { @content; } } .btn { @include button { color: blue; &:hover { color: red; } } } defines the mixin with @content inside and inside &:hover, then includes it with a block that passes normal and hover styles. This correctly applies custom hover styles.

3. Step 3: Why others fail

   @mixin button { padding: 1rem; border: none; @content; &:hover { @content; } } tries to use @content twice in the mixin without passing blocks properly. @mixin button { padding: 1rem; border: none; @content; } .btn:hover { @content; } applies hover styles outside the mixin, losing reusability. @mixin button { padding: 1rem; border: none; } .btn { @include button; &:hover { @content; } } misuses @content outside mixin context.

4. Final Answer:

   @mixin button { padding: 1rem; border: none; @content; &:hover { @content; } } .btn { @include button { color: blue; &:hover { color: red; } } } -> Option D

5. Quick Check:

   Mixin with @content block for hover = @mixin button { padding: 1rem; border: none; @content; &:hover { @content; } } .btn { @include button { color: blue; &:hover { color: red; } } } [OK]

Hint: Pass hover styles inside @include block with nested &:hover [OK]

Common Mistakes:

• Using @content outside mixin
• Trying to call @content multiple times without blocks
• Defining hover styles outside mixin losing flexibility