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Understand Your Tech Stack to Find and Fix Render-Blocking JS and CSS

Written by: Ozoemena Victor Tags: technical-seo

Published: Jan 19, 2026

Every millisecond your browser spends hunting for a script is a millisecond your visitor spends staring at a blank screen. And today with attention spans lower than ever, 

And in the year 2026, that "white ghost" of a blank web page is a conversion obstacle. 

This is because a blank screen triggers abandonment anxiety, in the three seconds it takes for your CSS to load, the user has already decided your site is broken and moved to a competitor.

To achieve a fast loading page that readers stay on, you'll need to work on improving your Core Web Vitals. One of the best places to make progress is to fix render-blocking JS and CSS. 

Understand your website’s tech stack first

Before you touch a single line of plugin or code, kindly ask yourself, what is the website created with?

Every site has its make-up or tech stack, the frameworks, CMS, libraries, and third-party scripts that make it work. Each of these can impact how quickly content appears to users and how efficiently search engines crawl your site. 

For instance;

• A site that is created on React, for example, may depend heavily on JavaScript to render content. 
• A site built on WordPress with multiple plugins, custom fonts, and widgets could load with extra scripts that block rendering. 
• And also third-party tools like analytics or ads can silently slow your page without warnings. 

Understanding your tech makeup is a strategic move for a strategic tech SEO person. It tells you where the real bottlenecks are, so every optimization you make is targeted, not random.

Check your tech stack with the BuiltWith Chrome extension

I use BuiltWith Chrome Extension because the tool lists exactly what you are dealing with and the next step(s). 

BuiltWith quickly shows you what powers a website:

• CMS
• Frameworks
• JavaScript libraries
• Third-party scripts

Why Builtwith works to identify render-blocking issues

1. You can spot heavy JavaScript frameworks like React or Angular. 
2. You can see which plugins or third-party tools may be slowing down your site.
3. It gives you a clear map of where to focus your optimization efforts.

Alternatives like Wappalyzer or WhatRuns do something similar, but BuiltWith is simple, fast, and effective. 

JavaScript and render-blocking style sheets can lead to bloat

Render-blocking JavaScript and CSS are scripts and stylesheets that a browser must load and process before it can display visible content, causing delays in page rendering.

This exercise may reveal that your site relies heavily on JavaScript, or there's not much JS across the site, and/or you could find extra JS in places that don't matter. 

For example, consider a landing page that stays blank for three seconds while the browser struggles to parse a massive JavaScript library just to animate a "Sign Up" button, a task that modern CSS can have handle near instantly.

Some assets create longer bottlenecks than others. For example, custom fonts can leave your text invisible for seconds. Third-party tracking scripts or live-chat widgets often force the entire page to pause while they attempt to connect to a distant, external server.

How browsers render a web page (HTML → CSS → JavaScript)

When someone clicks on a web page, and it opens, the browser doesn't load everything at once. It follows a sequence. 

The browser first reads the HTML, letting you know this is the structure of the pages- headings, images, and links. Without HTML, there's nothing to show. 

Next comes the CSS (Cascading Style Sheets): CSS tells your browser how that structure should look. From fonts, colours, layout, and spacing. At this point, your browser is ready or preparing to paint the page.

Then comes JavaScript, JS adds interactivity and behaviour. It takes care of things like pop-ups, forms, sliders, dynamic content, and tracking scripts. 

Understanding render-blocking through the browser's experience

The browser processes web pages through a series of steps known as the critical rendering path. This sequence converts HTML, CSS, and JavaScript into pixels on the screen.

Knowing these steps clarifies why certain resources block rendering.

• DOM Construction: The browser parses HTML to build the Document Object Model (DOM) according to web.dev.
  
  
• CSSOM Construction: CSS parsing creates the CSS Object Model (CSSOM) according to web.dev.
  
  
• Render Tree Creation: The DOM and CSSOM combine to form the render tree, which includes all visible elements as explained by web.dev.
  
  
• Layout: The browser calculates the position and dimensions of every visible element as noted by SitePoint.
  
  
• Paint: Pixels are rasterized and drawn to the screen per web.dev.

CSS is inherently render-blocking because the browser needs the style information to construct the render tree before it can paint the page correctly. 

According to SitePoint, synchronous JavaScript is parser-blocking and stops HTML parsing until it executes, particularly when it is placed in the `` without the `async` or `defer` attributes.

The user sees a blank page for longer, a result of an impacted LCP (largest contentful paint).

Signs your site may have render-blocking JS or CSS

If you are not a developer, there are signs to look for that JS or CSS is delaying your page load.

Keep an eye out for these;

1. PageSpeed or Lighthouse Warnings

Audits often flag ''Eliminate render-blocking resources'' for JS and CSS files.

These warnings highlight which scripts and styles are preventing your browser from painting content quickly. 

2. Delayed or blank content

If a page takes a few seconds longer than the usual time to show anything, it means the browser is waiting for JS or CSS to finish loading before rendering content. Even a small pause can affect SEO metrics like LCP, FCP (first contentful paint), and user experience. 

3. Heavy third-party scripts or styles

Analytics scripts, ads, chat widgets, or third-party style sheets can block rendering silently.

Even if your site’s main code is optimized, these external resources can slow the initial page paint.

Strategic fixes for render-blocking JS and CSS

Once you have identified the issue or problem, what JS or CSS is blocking rendering. You don't need to code yourself. There are high-level strategies that can make an impact. 

1. Defer or async non-critical JavaScript

Not all scripts need to load immediately. Scripts like analytics trackers, chat widgets, or other third-party tools can often be loaded after the main content.

Defer: Tells the browser to wait until the HTML is fully parsed before executing the script.

Async: Allows scripts to load in parallel without blocking page rendering.

This reduces the pause visitors see before your content appears.

2. Inline critical CSS

The bare minimum of styles required to instantly render the page's viewable area is known as critical CSS.

The browser may paint the page more quickly by inlining these styles in the <head> rather than loading a sizable CSS file separately. Without using JavaScript, this is one of the biggest speed boosts.

3. Remove unused JS and CSS

Plugins, themes, and outdated code are just a few of the scripts and styles that many websites gather over time.

Deleting these allows the browser to load fewer files, which speeds up rendering.

4. Optimize third-party scripts

Third-party scripts (ads, analytics, marketing tools) are often heavy and slow.

• Evaluate which scripts are essential.
• Load non-critical scripts after content.
• Use tools like BuiltWith to see which scripts may be affecting your load times.

Even small tweaks here can shave seconds off your page load.

Give it a try

Resolving render-blocking CSS and JavaScript can teach you so much about what’s happening under the hood. Like reading a car manual, you can now make precise tweaks, and the site performs a little bit better than it did before.

If you want to keep leveling up, explore our other educational pieces on interesting topics like:  Ahrefs vs Semrush: Should You Switch? Plus, How,

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