Raw HTML with v-html in Vue - Performance & Optimization

Performance: Raw HTML with v-html

MEDIUM IMPACT

Using v-html affects rendering speed and visual stability by injecting raw HTML directly into the DOM.

Displaying dynamic HTML content safely and efficiently

Vue

<div>{{ sanitizedText }}</div>

Rendering sanitized text instead of raw HTML avoids layout shifts and reduces reflows.

📈 Performance GainPrevents layout shifts and reduces reflows, improving CLS and rendering speed.

Displaying dynamic HTML content safely and efficiently

Vue

<div v-html="rawHtmlContent"></div>

Directly injecting raw HTML can cause layout shifts and security risks if content changes often or is untrusted.

📉 Performance CostTriggers layout recalculation and repaint on each content update, causing CLS and slower rendering.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Using v-html with dynamic content	Inserts multiple nodes dynamically	Triggers reflow on each update	High paint cost due to layout shifts	[X] Bad
Rendering sanitized text without v-html	Minimal DOM changes	No reflows from HTML injection	Low paint cost	[OK] Good

Rendering Pipeline

v-html inserts raw HTML into the DOM, causing the browser to recalculate styles and layout for the new nodes, then repaint and composite the updated content.

→Style Calculation

→Layout

→Paint

→Composite

⚠️ BottleneckLayout stage is most expensive due to recalculations triggered by injected HTML.

Core Web Vital Affected

CLS

Using v-html affects rendering speed and visual stability by injecting raw HTML directly into the DOM.

Optimization Tips

1Avoid frequent updates to content rendered with v-html to reduce layout recalculations.

2Sanitize HTML content before injecting to prevent security risks and unexpected layout changes.

3Prefer rendering plain text or Vue templates over raw HTML when possible for better performance and stability.

Performance Quiz - 3 Questions

What is a main performance risk when using v-html with frequently changing content?

A Increased JavaScript bundle size

B Slower network requests

C Frequent layout recalculations causing layout shifts

D Reduced accessibility

DevTools: Performance

How to check: Record a performance profile while updating v-html content and observe layout and paint events.

What to look for: Look for frequent layout recalculations and large paint times indicating costly reflows from raw HTML injection.

Practice

1. What does the v-html directive do in Vue?

easy

A. It listens for user input events.

B. It binds a JavaScript object to the element.

C. It inserts raw HTML content inside an element.

D. It applies CSS styles dynamically.

Solution

1. Step 1: Understand the purpose of v-html

   The v-html directive is used to insert raw HTML content inside an element in Vue.

2. Step 2: Compare with other options

   Other options describe different Vue features like event listening or styling, which are unrelated to v-html.

3. Final Answer:

   It inserts raw HTML content inside an element. -> Option C

4. Quick Check:

   v-html = inserts raw HTML [OK]

Hint: Remember: v-html inserts raw HTML inside elements [OK]

Common Mistakes:

• Confusing v-html with event binding
• Thinking v-html applies styles
• Assuming v-html binds JavaScript objects

2. Which of the following is the correct syntax to use v-html in a Vue template?

easy

A.

B.

C.

D.

Solution

1. Step 1: Identify correct directive syntax

   The correct directive to insert raw HTML is v-html, used as v-html="rawHtml".

2. Step 2: Check other options for correctness

   uses a non-existent directive :html.

   uses v-text which inserts plain text, not HTML.

   uses an invalid directive v-bind-html.

3. Final Answer:

   <div v-html="rawHtml"></div> -> Option B

4. Quick Check:

   Correct syntax = v-html="variable" [OK]

Hint: Use v-html="variable" to insert raw HTML [OK]

Common Mistakes:

• Using v-text instead of v-html
• Trying v-bind-html which is invalid
• Using shorthand :html which doesn't exist

3. Given the Vue template and data below, what will be rendered inside the div?

<template>
  <div v-html="htmlContent"></div>
</template>

<script setup>
const htmlContent = '<strong>Hello</strong> <em>Vue</em>!'
</script>

medium

A. Hello Vue! (formatted with bold and italic)

B. <strong>Hello</strong> <em>Vue</em>! (raw text with tags visible)

C. Hello Vue! (plain text without formatting)

D. An error occurs because of invalid HTML

Solution

1. Step 1: Understand v-html rendering

   The v-html directive inserts the HTML string as actual HTML, so tags like <strong> and <em> become formatted text.

2. Step 2: Analyze the given HTML string

   The string contains <strong>Hello</strong> and <em>Vue</em>, so the output will show "Hello" in bold and "Vue" in italic.

3. Final Answer:

   <strong>Hello</strong> <em>Vue</em>! (formatted with bold and italic) -> Option A

4. Quick Check:

   v-html renders HTML tags as formatted content [OK]

Hint: v-html renders HTML tags as formatted content, not plain text [OK]

Common Mistakes:

• Expecting raw HTML tags to show as text
• Confusing v-html with v-text
• Assuming an error occurs with HTML tags

4. What is the main issue with this Vue template?

<template>
  <div v-html=htmlContent></div>
</template>

<script setup>
const htmlContent = '<p>Hello</p>'
</script>

medium

A. The v-html directive is missing quotes around the binding expression.

B. The htmlContent variable is not reactive.

C. The HTML string is invalid and causes an error.

D. v-html cannot be used inside a <div> element.

Solution

1. Step 1: Check v-html syntax

   The directive value must be quoted, like v-html="htmlContent". Here it is missing quotes.

2. Step 2: Verify other options

   The variable is valid and reactive by default. The HTML string is valid. v-html works on any element including div.

3. Final Answer:

   The v-html directive is missing quotes around the binding expression. -> Option A

4. Quick Check:

   v-html value must be quoted [OK]

Hint: Always put quotes around v-html binding expressions [OK]

Common Mistakes:

• Omitting quotes around v-html value
• Thinking v-html only works on certain tags
• Assuming HTML string causes errors

5. You want to display user-generated HTML safely using v-html. Which approach helps prevent security risks?

hard

A. Use v-html directly without any checks.

B. Replace v-html with v-text to render HTML tags.

C. Escape all HTML tags and then use v-html.

D. Sanitize the HTML string before binding it to v-html.

Solution

1. Step 1: Understand security risks of v-html

   Using v-html with user input can cause cross-site scripting (XSS) attacks if the HTML is not safe.

2. Step 2: Identify safe usage

   Sanitizing the HTML string removes dangerous code before inserting it, making it safe to use with v-html.

3. Step 3: Evaluate other options

   Using v-html directly is unsafe. Escaping tags then using v-html defeats the purpose. Using v-text shows raw text, not HTML.

4. Final Answer:

   Sanitize the HTML string before binding it to v-html. -> Option D

5. Quick Check:

   Sanitize user HTML before v-html to prevent XSS [OK]

Hint: Always sanitize user HTML before using v-html [OK]

Common Mistakes:

• Using v-html directly with user input
• Escaping HTML then using v-html (no effect)
• Confusing v-text with v-html for HTML rendering