How Lazy Load Image Tester Works

This tool supports two workflows: an HTML analyzer that scans your markup for image tags and lazy-loading signals, and a test page generator that builds an offline HTML file you can open in a browser to observe network activity while scrolling. Both workflows are designed to be practical: you can paste real code from your project or use a list of image URLs to reproduce issues in a controlled environment.

The analyzer focuses on what you can verify from markup alone: attributes like loading, common data attributes used by lazy loaders, and quality indicators such as alt text and intrinsic sizing. The generator focuses on observable behavior: you can open the file, throttle your network, and confirm whether requests start before the user reaches each image.

Step-by-Step

• 1) Choose a mode: Pick Analyze HTML to audit markup, or Generate Test Page to build a scrollable demo file.
• 2) Provide input: Paste the HTML snippet you want to audit, or paste one image URL per line for the generator.
• 3) Configure options: For the test page, choose native loading="lazy", an IntersectionObserver-based swap, or eager loading. Adjust spacer height to force scrolling.
• 4) Generate output: The tool produces a copy-ready report or a complete HTML document you can download.
• 5) Validate in your browser: Open DevTools, throttle the network, and scroll to confirm when requests fire and whether placeholders reserve space.
• 6) Apply fixes: Update your templates (attributes, dimensions, responsive sources), then re-test to confirm the change improved behavior rather than simply shifting the problem.

Key Features

HTML Markup Audit

Paste HTML from a template, CMS, or rendered page source and instantly see how many images use native lazy loading, how many rely on data attributes (like data-src), and how many have no lazy-loading hint at all. The report also highlights missing alt text and missing width/height attributes—both common issues that can impact accessibility and layout stability.

This is especially helpful when markup is generated by components or shortcodes. You can confirm whether a refactor changed attributes, whether an image helper stopped emitting dimensions, or whether a “lazy” class is present without the script that’s supposed to handle it.

Pattern Detection for Popular Lazy-Loading Approaches

Modern sites mix strategies: native lazy loading, custom IntersectionObserver logic, and third-party libraries that swap src/srcset. The analyzer flags common keywords such as “lazysizes” or “IntersectionObserver” so you can quickly confirm what your page is likely doing without reading every script file.

Because the output is text-based, it’s easy to paste into a code review or a performance ticket. That way, the team can agree on what the page is doing today before debating what it should do tomorrow.

Scroll Test Page Generator

When you need a reproducible test case, the generator creates a standalone HTML file with your chosen loading method. It inserts spacer blocks between images so you can force content below the fold and verify lazy loading under fast scroll, slow networks, or CPU throttling—exactly the conditions that uncover blank gaps or delayed loads.

Generated pages are intentionally simple: no build steps, no external dependencies, and no hidden state. If you can open an HTML file, you can test. This makes it easy to share the test case with teammates, clients, or QA and compare results across browsers.

Copy and Download Output

Reports and generated HTML are presented in a single output panel. Copy the result into a ticket, a pull request, or documentation, or download it as a file (TXT for audits, HTML for test pages). This keeps your workflow simple and avoids extra formatting steps.

Practical Recommendations

The audit output includes guidance you can apply immediately: avoid lazy-loading your LCP image, reserve layout space with dimensions, keep placeholders lightweight, and use responsive image attributes responsibly. The focus is on changes you can implement in templates and verify quickly, rather than theoretical advice that requires a full tooling overhaul.

Use Cases

• Debugging “images load too late” complaints: Generate a scroll test page and reproduce the issue with throttling to see whether requests start on time.
• Auditing templates after a redesign: Paste new HTML to confirm loading="lazy" didn’t get removed, and check for missing width/height that can cause CLS.
• Reviewing third-party widgets: Identify whether embedded HTML relies on data-src patterns without a robust script fallback.
• Preparing performance fixes: Compare counts of native lazy vs. non-lazy images and prioritize pages where below-the-fold images are still eager-loaded.
• Quality assurance before release: Use the tool as a quick checklist: alt text present, dimensions present, and lazy-loading applied only where it makes sense.
• Teaching and demos: Share the generated test page with colleagues to demonstrate the difference between native lazy loading and IntersectionObserver swaps.
• Regression checks after framework upgrades: Confirm that server-side rendering or component refactors didn’t change how images are emitted.

Whether you are a developer optimizing an app, an SEO specialist reviewing page speed, or a designer validating a component library, this tool helps you move from “I think images are lazy-loaded” to “I can prove exactly how and when they load.” It’s also useful when you’re comparing approaches: if native lazy loading starts too late for your design, an observer-based solution with a larger root margin might provide a better balance between savings and smooth scrolling.

Optimization Tips

Don’t Lazy-Load the LCP Image

The largest contentful paint (LCP) element is often a hero image. If you lazy-load it, the browser may delay fetching the asset, hurting your LCP metric and making the page feel slower. Treat lazy loading as a below-the-fold optimization, and keep above-the-fold imagery eager with good caching, compression, and responsive sources.

Reserve Space to Prevent Layout Shift

Missing width/height attributes (or equivalent CSS sizing) can cause content to jump when images load. Even when lazy loading is correct, CLS issues can harm user experience. Add intrinsic dimensions, use aspect-ratio boxes where appropriate, and keep placeholders visually stable so the page doesn’t reflow during scrolling.

Test Under Realistic Conditions

Lazy loading failures often appear only with fast scrolling, slow networks, or heavy main-thread work. Use the generated test page with DevTools throttling and CPU slowdown. If images are still loading too late, increase IntersectionObserver rootMargin so requests start earlier, or confirm that native lazy loading is not being overridden by scripts and classes.

FAQ