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HTMLmarkup~8 mins

What are attributes in HTML - Performance Impact

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Performance: What are attributes
LOW IMPACT
Attributes affect how the browser interprets and renders HTML elements, impacting initial load and rendering speed.
Adding attributes to HTML elements
HTML
<img src="image.jpg" alt="Descriptive text" width="600" height="400" loading="lazy">
Specifying dimensions and lazy loading helps browser reserve space and defer offscreen images.
📈 Performance GainReduces layout shifts and speeds up Largest Contentful Paint
Adding attributes to HTML elements
HTML
<img src="image.jpg" alt="" width="" height="" loading="eager">
Empty or unnecessary attributes cause extra parsing and can delay image loading.
📉 Performance CostBlocks rendering until image loads; triggers layout recalculation if dimensions are missing
Performance Comparison
PatternDOM OperationsReflowsPaint CostVerdict
Empty or missing size attributes on images1 DOM nodeTriggers reflow on image loadHigh paint cost due to layout shift[X] Bad
Images with width and height attributes1 DOM nodeSingle reflow reserved spaceLower paint cost, stable layout[OK] Good
Rendering Pipeline
Attributes are parsed during HTML parsing and influence style calculation, layout, and paint stages.
HTML Parsing
Style Calculation
Layout
Paint
⚠️ BottleneckLayout stage when attributes affect element size or visibility
Core Web Vital Affected
LCP
Attributes affect how the browser interprets and renders HTML elements, impacting initial load and rendering speed.
Optimization Tips
1Always specify width and height for images to prevent layout shifts.
2Use loading="lazy" on images to defer offscreen loading and improve LCP.
3Avoid empty or redundant attributes to reduce parsing and rendering overhead.
Performance Quiz - 3 Questions
Test your performance knowledge
How do missing width and height attributes on images affect page performance?
AThey have no effect on performance
BThey improve loading speed by deferring image size calculation
CThey cause layout shifts and delay rendering
DThey reduce paint cost by skipping layout
DevTools: Performance
How to check: Record a page load and look for layout shifts and image load timing in the flame chart.
What to look for: Look for layout shifts caused by images without size attributes and long image load blocking