While it's common practice to keep JavaScript at the bottom of the HTML document, does the same rule apply for CSS as well?
I understand that we can't place external CSS files outside the HTML document.
While it's common practice to keep JavaScript at the bottom of the HTML document, does the same rule apply for CSS as well?
I understand that we can't place external CSS files outside the HTML document.
CSS and JavaScript impact rendering differently
In the words of Yahoo's Developer Network Blog
Stylesheets delay progressive rendering until fully downloaded, hence it is advisable to place them in the document HEAD for faster rendering. On the other hand, scripts block progressive rendering below them. Placing scripts towards the end of the page allows more content to render sooner.
Furthermore, adding CSS to the head ensures that styling begins instantly, preventing a Flash Of Unstyled Content that occurs when style tags are placed at the bottom of lengthy HTML pages.
Unlike Javascript, CSS does not obstruct the rendering process. However, some browsers may exhibit unexpected behavior if CSS is not placed within the <head> section. Certain browsers might ignore the CSS rules entirely or apply them inconsistently. It's worth noting that HTML5 restricts the use of <style> outside the <head> element, unless the "scoped" attribute is utilized (although this feature is currently unsupported by most browsers).
When it comes to loading CSS stylesheets, they are typically loaded using the <link> element, which is supposed to be placed in the head section of the document. Contrary to common belief, CSS does not block HTML rendering because it gets applied after the browser has finished loading, just like any other <link> elements. On the other hand, JavaScript can indeed block HTML rendering as the browser assumes that JS may need to manipulate the DOM before it's fully loaded, causing delays.
Imagine if CSS was loaded the same way as JS - none of the elements on the page would be styled correctly as they wouldn't have been loaded yet. This is why CSS is applied after the initial load and doesn't hinder the loading process.
CSS is immediately applied to the DOM element as soon as the browser reads the CSS file.
However, when it comes to JavaScript, you have the flexibility to keep the script at hand and make it run after all HTML has finished loading. For example:
window.onload = function (){
//we begin here
}
JavaScript
JS has the power to modify your web page, causing browsers to wait for all external JS files to be downloaded, interpreted, and executed before proceeding with the rest of the HTML content that follows a <script src="..." >.
To ensure a smoother user experience, it is recommended to place external JS scripts at the bottom of the <body>. This allows for quicker rendering of the HTML content, giving users the impression that something is happening on the page.
Cascading Style Sheets (CSS)
In contrast, CSS cannot manipulate the DOM or make significant changes to the page layout, so browsers do not delay in downloading, parsing the remaining HTML content, and progressively rendering the page as they do with JS. While it may appear to block rendering, placing CSS at the top can help prevent the dreaded FOUC (Flash Of Unstyled Content). However, CSS does not delay download.
Interestingly, due to the prevalence of mobile devices, simply putting CSS in the HEAD and JS at the bottom may no longer suffice. Consider including above-the-fold (ATF) CSS inline, and concatenating the remainder of your minified CSS with your JS at the end or using deferred and async techniques for loading.
For more information, check out:
What are the benefits of placing Javascript code at the bottom of a web page?
In my opinion, it is recommended to segregate CSS and Javascript into separate files and link them within the <head> section of your HTML document.
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