How to Test a Website at Different Screen Sizes
A beginner, hands-on guide to test a website at different screen sizes with browser inspection, exact code-level changes and verification.
By the end: You will implement and verify test a website at different screen sizes on a real local/staging page.
You will implement and verify test a website at different screen sizes on a real local/staging page.
Key terms before you start
These definitions make the steps easier to follow and help distinguish controls, files and concepts that can look similar at first.
What you need before starting
- A page you are allowed to edit
- Access to the HTML/CSS/JS or CMS/template that renders it
- A modern browser with Developer Tools
Step-by-step tutorial
Every step is shown below. Work through them in order and use each checkpoint to confirm the result before moving on.
Work on a local or staging copy
Open the project containing the website source. If this is a production site, use a staging copy or backup before structural changes.
A browser page is rendered from source files/CMS output; DevTools edits do not save back to the server.
Move it into the actual source and reload.
Open the page in a modern browser
Load the page, then press F12 or Ctrl+Shift+I (Windows/Linux) to open Developer Tools.
DevTools lets you inspect the DOM, applied CSS, computed sizes, requests and console errors.
Move it into the actual source and reload.
Open responsive mode
DevTools → Toggle device toolbar (often Ctrl+Shift+M after DevTools opens).
Presets are useful starting points, not the complete test.
Move it into the actual source and reload.
Test 320–360px range
Check narrow layout, navigation, forms, long words and buttons.
Small widths expose rigid fixed dimensions.
Drag continuously to desktop
Slowly widen the viewport.
Bugs often occur between named presets.
Test landscape-like short height
Reduce viewport height and open menus/modals.
Tall overlays can trap controls below viewport.
Test 200% browser zoom
Zoom while using desktop width.
Zoom can approximate low-vision text enlargement and reveal rigid layouts.
Test content states
Open validation errors, dropdowns, cookie banners, accordions and long headings.
Empty/default states are not enough.
Use at least one real phone
Open the staging/public URL on real hardware.
Emulation cannot reproduce every mobile browser keyboard/touch behavior.
Check caching, loaded asset URL, theme/template and logged-out view.
Test the public/staging URL logged out
Open the final URL in a private/incognito window.
CMS/admin sessions can hide caching, permission or styling differences.
Check caching, loaded asset URL, theme/template and logged-out view.
Document the changed file/rule
Record the file/template/component and purpose of the change in the project notes or version-control commit.
Future maintainers should not have to reverse-engineer why a rule exists.
Check caching, loaded asset URL, theme/template and logged-out view.
Reload the real page and reproduce the result after all temporary DevTools edits are gone.
Concrete examples
Troubleshooting
Move it into the actual source and reload.
Check caching, loaded asset URL, theme/template and logged-out view.
Use the Styles/Computed panels to trace the final value instead of adding stronger selectors blindly.
The change has a traceable reason/location.
Continue with these tutorials
Documentation and references
Use these primary and supporting sources to verify current controls, browser behavior and product-specific details.