HTML rendering with {@html} in Svelte - Performance & Optimization

Performance: HTML rendering with {@html}

MEDIUM IMPACT

This affects how quickly the browser can render dynamic HTML content and impacts layout stability and interaction responsiveness.

Rendering dynamic HTML content safely and efficiently

Svelte

<div>{#if safeHtml}<div>{@html safeHtml}</div>{/if}</div>

Conditionally rendering sanitized HTML reduces unexpected layout shifts and improves visual stability.

📈 Performance GainReduces reflows and CLS by controlling when HTML is injected

Rendering dynamic HTML content safely and efficiently

Svelte

<div>{@html dynamicHtml}</div>

Directly injecting raw HTML can cause layout shifts if content size changes after initial render and may introduce security risks if content is not sanitized.

📉 Performance CostTriggers multiple reflows if content size changes; can cause CLS issues

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Direct {@html} injection without control	Inserts raw nodes dynamically	Multiple reflows if content changes	High paint cost due to layout shifts	[X] Bad
Conditional rendering with sanitized HTML	Controlled DOM insertion	Single or no reflows if stable	Lower paint cost with stable layout	[OK] Good

Rendering Pipeline

When {@html} is used, the browser parses the injected HTML string and inserts it into the DOM, triggering style recalculation and layout if the content size changes.

→DOM Construction

→Style Calculation

→Layout

→Paint

→Composite

⚠️ BottleneckLayout stage is most expensive due to reflows caused by dynamic content size changes.

Core Web Vital Affected

CLS

This affects how quickly the browser can render dynamic HTML content and impacts layout stability and interaction responsiveness.

Optimization Tips

1Avoid injecting large or frequently changing raw HTML with {@html} to reduce layout shifts.

2Sanitize and stabilize HTML content before using {@html} to improve visual stability.

3Use conditional rendering to control when {@html} content is inserted to minimize reflows.

Performance Quiz - 3 Questions

What is a main performance risk when using {@html} in Svelte?

A Increasing JavaScript bundle size significantly

B Blocking network requests during rendering

C Causing layout shifts due to dynamic content size changes

D Preventing CSS from applying styles

DevTools: Performance

How to check: Record a performance profile while the component renders dynamic HTML with {@html}. Look for layout and paint events after HTML injection.

What to look for: Look for layout thrashing and long layout times indicating reflows; check for CLS warnings in the summary.

Practice

1. What does the {@html} tag do in a Svelte component?

easy

A. It escapes HTML tags and shows them as plain text.

B. It renders a string as actual HTML inside the component.

C. It creates a new HTML element dynamically.

D. It comments out HTML code so it is not rendered.

Solution

1. Step 1: Understand {@html} purpose

   The {@html} tag tells Svelte to treat the string as HTML, not plain text.

2. Step 2: Compare with normal rendering

   Normally, Svelte escapes HTML tags to show them as text, but {@html} renders them as real HTML.

3. Final Answer:

   It renders a string as actual HTML inside the component. -> Option B

4. Quick Check:

   {@html} = renders HTML [OK]

Hint: Remember {@html} outputs HTML, not plain text [OK]

Common Mistakes:

• Thinking {@html} escapes HTML instead of rendering it
• Confusing {@html} with creating new elements
• Assuming {@html} comments out code

2. Which of the following is the correct syntax to render HTML stored in a variable htmlContent inside a Svelte component?

easy

A. <div>{@html htmlContent}</div>

B. <div>{@html(htmlContent)}</div>

C. <div>{htmlContent}</div>

D. <div>{#html htmlContent}{/html}</div>

Solution

1. Step 1: Recall correct {@html} syntax

   The correct syntax is {@html variableName} without parentheses or block tags.

2. Step 2: Check each option

   <div>{htmlContent}</div> renders as plain text, B uses invalid parentheses, D uses wrong block syntax.

3. Final Answer:

   <div>{@html htmlContent}</div> -> Option A

4. Quick Check:

   Correct {@html} syntax = {@html variable} [OK]

Hint: Use {@html variable} without parentheses or blocks [OK]

Common Mistakes:

• Adding parentheses around variable in {@html}
• Using block syntax like {#html}{/html}
• Not using {@html} and rendering raw string

3. Given this Svelte code:

  <script>
    let htmlString = '<strong>Hello</strong> <em>World</em>';
  </script>

  <div>{@html htmlString}</div>

What will be the rendered output in the browser?

medium

A. Hello World (plain text, no formatting)

B. <strong>Hello</strong> <em>World</em> (raw text)

C. Hello World (formatted text)

D. An error occurs because of unsafe HTML

Solution

1. Step 1: Understand {@html} rendering

   {@html renders the string as HTML, so tags become real formatting.

2. Step 2: Analyze the string content

   The string contains <strong> and <em> tags, so output will be bold and italic text.

3. Final Answer:

   <strong>Hello</strong> <em>World</em> (formatted text) -> Option C

4. Quick Check:

   HTML string renders as formatted text [OK]

Hint: {@html} renders tags as formatting, not raw text [OK]

Common Mistakes:

• Expecting tags to show as text instead of formatting
• Assuming plain text output without tags
• Thinking unsafe HTML causes runtime error

4. What is the main problem with this Svelte code?

  <script>
    let unsafeHtml = '';
  </script>

  <div>{@html unsafeHtml}</div>

medium

A. It can run unsafe scripts and cause security issues (XSS).

B. Svelte automatically sanitizes the HTML, so no problem.

C. The image will not display because of invalid src attribute.

D. It will cause a syntax error because of quotes inside the string.

Solution

1. Step 1: Identify unsafe HTML usage

   The string contains an image with an onerror event that runs JavaScript.

2. Step 2: Understand {@html} risks

   {@html renders raw HTML, so malicious scripts can run causing XSS security issues.

3. Final Answer:

   It can run unsafe scripts and cause security issues (XSS). -> Option A

4. Quick Check:

   {@html with unsafe HTML = security risk [OK]

Hint: Never use {@html} with untrusted content [OK]

Common Mistakes:

• Thinking Svelte sanitizes HTML automatically
• Assuming syntax error due to quotes inside string
• Believing image won't display due to src

5. You want to display user-generated content that may contain HTML tags safely in your Svelte app. Which approach is best?

hard

A. Wrap the user content in a <pre> tag and use {@html}.

B. Escape the user content and render it normally without {@html}.

C. Use {@html} directly on the user content to render formatting.

D. Sanitize the user content to remove unsafe tags, then use {@html}.

Solution

1. Step 1: Understand risks of raw {@html}

   Using {@html} directly on user content risks XSS attacks if content is unsafe.

2. Step 2: Best practice for safe HTML rendering

   Sanitize user content to remove dangerous tags/scripts, then render with {@html} for formatting.

3. Final Answer:

   Sanitize the user content to remove unsafe tags, then use {@html}. -> Option D

4. Quick Check:

   Sanitize first, then {@html} = safe rendering [OK]

Hint: Always sanitize user HTML before {@html} rendering [OK]

Common Mistakes:

• Using {@html} directly on raw user input
• Escaping content but losing formatting
• Using <pre> which shows raw text, not formatted HTML