Correct Way to Test Responsive Behavior With Conditional CSS Delivery

If you manually resize your desktop browser to check responsiveness and see layout issues, that does not reflect how real visitors experience your site. Visitors do not change their device screen size while a page is open. Modern performance optimizations deliver different CSS for different device classes to reduce file size and speed up loading. This is expected and beneficial behavior.

Many performance plugins and platforms use Conditional CSS Delivery with critical CSS generation and media query splitting. When a page is requested:

• The system detects the device class, for example desktop or mobile.

• It parses your stylesheets, maps selectors to breakpoints, and builds a device specific CSS bundle.

• Desktop visitors receive only desktop rules. Mobile specific CSS is excluded for them.

• Result: fewer bytes over the network, faster first render, and better Core Web Vitals.

Because the CSS bundle is chosen at page load time, it does not change mid session if you drag your desktop window narrower or wider. The page will continue using the CSS that matched the original viewport at load.

When you load a page on a wide desktop window, you receive the desktop CSS bundle. If you then manually shrink the window to a mobile width without reloading, the page still uses the desktop bundle. Elements that depend on mobile only rules may not apply until you refresh. This is normal for setups that split CSS by device to improve performance.

Use one of the following methods to simulate real user behavior and receive the correct CSS bundle for each device class.

1. Real device testing

• Open the page on a physical phone or tablet.

• Refresh once after the page loads to ensure a fresh cache.

2. Browser responsive device mode

• Open DevTools and select a specific device or width.

• Reload the page after switching to a new device size to fetch the matching CSS bundle.

• Test each breakpoint by switching, then reloading again.

3. Separate windows or sessions

• Keep one tab sized for desktop and another for mobile.

• Reload each tab before evaluating layout and interactions.

Some experiences do change layout during a session, for example:

• Device orientation changes on mobile

• App like UIs that rely on live resize events

• Embedded builders or editors

If your site requires live resizing in the same session, talk to your developer about loading a unified CSS bundle or adding a small supplemental stylesheet for live resize edge cases. Note that this may increase CSS size and reduce performance.

Will real users see these issues on phones?
No. A mobile visitor loads the mobile bundle at the start, so the correct rules apply without manual resizing.

Why do we split CSS at all?
Smaller CSS equals faster rendering, less blocking time, and better Core Web Vitals. Sending only what a device needs is a proven optimization.

Do I always need to refresh after changing viewport in DevTools?
Yes. Switching device size without a reload can mix the wrong bundle with the new width.

What about caching and CDNs?
Edge caches can also vary content by device class. Reloading after switching sizes ensures you request the correct variant from cache.

• Reproduce on a real device and confirm the issue still exists.

• In DevTools, pick a device size, reload, then test.

• Clear the browser cache or use a hard reload to discard stale bundles.

• Disable browser extensions that inject CSS or scripts during testing.

• If the layout only breaks when resizing without a reload, it is expected with conditional delivery and not a user facing defect.

• Document the device, viewport width, and that you reloaded after switching sizes.

• Provide screenshots or screen recordings captured after the reload.

• Distinguish between issues seen only during manual resizing and issues reproducible on first load for that device width.

• Our platform and optimization plugins may use device aware CSS bundling to improve performance.

• If a component must elegantly handle mid session resizes, consider lightweight runtime utilities or a small supplemental CSS that covers critical differences, balancing performance with flexibility.

• Avoid undoing conditional delivery globally unless there is a strong product requirement, since it increases CSS size and harms speed.

Key takeaway: Manual desktop resizing without a reload is not representative of real usage when conditional CSS delivery is enabled. Always reload after changing viewport or test on real devices to evaluate the true visitor experience.