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20 Tips for Optimizing CSS Performance — SitePoint

A Friendly Introduction to Flexbox for Beginners

On this article, we take a look at 20 methods to optimize your CSS in order that it’s faster-loading, simpler to work with and extra environment friendly.

In accordance with the newest HTTP Archive stories, the online stays a bloated mess with the legendary median web site requiring 1,700Kb of knowledge cut up over 80 HTTP requests and taking 17 seconds to completely load on a cellular gadget.

The Full Information to Decreasing Web page Weight supplies a variety of strategies. On this article, we’ll consider CSS. Admittedly, CSS is never the worst offender and a typical website makes use of 40KB unfold over 5 stylesheets. That stated, there are nonetheless optimizations you can also make, and methods to vary how we use CSS that may increase website efficiency.

You’ll be able to’t handle efficiency issues until you realize the place the faults lie. Browser DevTools are the most effective place to start out: launch from the menu or hit F12, Ctrl + Shift + I or Cmd + Alt + I for Safari on macOS.

All browsers supply comparable amenities, and the instruments will open slowly on badly-performing pages! Nevertheless, probably the most helpful tabs embrace the next …

The Community tab shows a waterfall graph of belongings as they obtain. For greatest outcomes, disable the cache and think about throttling to a decrease community velocity. Look for information which are sluggish to obtain or block others. The browser usually blocks browser rendering whereas CSS and JavaScript information obtain and parse.

The Performance tab analyses browser processes. Begin recording, run an exercise reminiscent of a web page reload, then cease recording to view the outcomes. Look for:

  1. Extreme format/reflow actions the place the browser has been pressured to recalculate the place and measurement of web page parts.
  2. Costly paint actions the place pixels are modified.
  3. Compositing actions the place the painted elements of the web page are put collectively for displaying on-screen. That is usually the least processor-intensive motion.

Chrome-based browsers present an Audits tab which runs Google’s Lighthouse software. It’s typically utilized by Progressive Net App builders, but in addition makes CSS efficiency ideas.

On-line Choices

Alternatively, use on-line evaluation instruments that aren’t influenced by the velocity and capabilities of your system and community. Most can check from various places around the globe:

2. Make Huge Wins First

CSS is unlikely to be the direct explanation for efficiency points. Nevertheless, it might load heavy-hitting belongings which may be optimized inside minutes. Examples:

  • Activate HTTP/2 and GZIP compression in your server
  • Use a content material supply community (CDN) to extend the variety of simultaneous HTTP connections and replicate information to different places around the globe
  • Take away unused information.

Photographs are usually the most important explanation for web page bulk, but many websites fail to optimize successfully:

  1. Resize bitmap pictures. An entry-level smartphone will take multi-megapixel photographs that may’t be displayed in full on the most important HD display. Few websites would require photographs of greater than 1,600 pixels in width.
  2. Make sure you use an applicable file format. Sometimes, JPG is greatest for pictures, SVG for vector pictures, and PNG for the whole lot else. You possibly can experiment to seek out the optimum sort.
  3. Use picture instruments to scale back file sizes by striping metadata and growing compression elements.

That stated, remember that xKb of picture knowledge just isn’t equal to xKb of CSS code. Binary photographs obtain in parallel and require little processing to put on a web page. CSS blocks rendering and have to be parsed into an object mannequin earlier than the browser can proceed.

three. Exchange Photographs with CSS Results

It’s not often needed to make use of background pictures for borders, shadows, rounded edges, gradients and a few geometric shapes. Defining an “image” utilizing CSS code makes use of significantly much less bandwidth and is simpler to switch or animate later.

four. Take away Pointless Fonts

Providers reminiscent of Google Fonts make it straightforward so as to add customized fonts to any web page. Sadly, a line or two of code can retrieve lots of of kilobytes of font knowledge. Suggestions:

  1. Solely use the fonts you want.
  2. Solely load the weights and types you require — for instance, roman, 400 weight, no italics.
  3. The place attainable, restrict the character units. Google fonts lets you decide sure characters by including a &textual content= worth to the font URL — reminiscent of content=SitePon for displaying “SitePoint” in Open Sans.
  4. Contemplate variable fonts, which outline a number of weights and types by interpolation so information are smaller. Help is presently restricted to Chrome, Edge, and a few editions of Safari however ought to develop quickly. See Find out how to Use Variable Fonts.
  5. Contemplate OS fonts. Your 500Kb net font could also be on-brand, however would anybody discover for those who switched to the generally out there Helvetica or Arial? Many websites use customized net fonts, so commonplace OS fonts are significantly much less widespread than they have been!

5. Keep away from @import

The @import at-rule permits any CSS file to be included inside one other. For instance:

/* foremost.css */
@import url(“base.css”);
@import url(“layout.css”);
@import url(“carousel.css”);

This seems an inexpensive method to load smaller elements and fonts. It’s not. @import guidelines could be nested so the browser should load and parse every file in collection.

A number of <hyperlink> tags inside the HTML will load CSS information in parallel, which is significantly extra environment friendly — particularly when utilizing HTTP/2:

<hyperlink rel=”stylesheet” href=”base.css”>
<hyperlink rel=”stylesheet” href=”layout.css”>
<hyperlink rel=”stylesheet” href=”carousel.css”>

That stated, there could also be extra preferable choices …

6. Concatenate and Minify

Most construct instruments can help you mix all partials into one giant CSS file that has pointless whitespace, feedback and characters eliminated.

Concatenation is much less mandatory with HTTP/2, which pipelines and multiplexes requests. In some instances, separate information could also be useful when you have smaller, regularly-changing CSS belongings. Nevertheless, most websites are more likely to profit from sending a single file that’s instantly cached by the browser.

Minification might not deliver appreciable advantages when you will have GZIP enabled. That stated, there’s no actual draw back.

Lastly, you possibly can contemplate a construct course of that orders properties persistently inside declarations. GZIP can maximize compression when generally used strings are used all through a file.

7. Use Trendy Format Methods

For a few years it was vital to make use of CSS float to put out pages. The method is a hack. It requires plenty of code and margin/padding tweaking to make sure layouts work. Even then, floats will break at smaller display sizes until media queries are added.

The fashionable options:

  • CSS Flexbox for one-dimensional layouts which (can) wrap to the subsequent row based on the widths of every block. Flexbox is right for menus, picture galleries, playing cards, and so on.
  • CSS Grid for two-dimensional layouts with specific rows and columns. Grid is right for web page layouts.

Each choices are easier to develop, use much less code, can adapt to any display measurement, and render quicker than floats as a result of the browser can natively decide the optimum format.

eight. Scale back CSS Code

Probably the most dependable and quickest code is the code you want by no means write! The smaller your stylesheet, the faster it’s going to obtain and parse.

All builders begin with good intentions, however CSS can bloat over time because the function rely will increase. It’s simpler to retain previous, pointless code fairly than take away it and danger breaking one thing. A couple of suggestions to think about:

  • Be cautious of huge CSS frameworks. You’re unlikely to make use of a big proportion of the types, so solely add modules as you want them.
  • Arrange CSS into smaller information (partials) with clear duties. It’s simpler to take away a carousel widget if the CSS is clearly outlined in widgets/_carousel.css.
  • Think about naming methodologies resembling BEM to assist the event of discrete elements.
  • Keep away from deeply nested Sass/preprocessor declarations. The expanded code can turn into unexpectedly giant.
  • Keep away from utilizing !essential to override the cascade.
  • Keep away from inline types in HTML.

Instruments resembling UnCSS will help take away redundant code by analyzing your HTML, however be cautious about CSS states brought on by JavaScript interplay.

9. Cling to the Cascade!

The rise of CSS-in-JS has allowed builders to keep away from the CSS international namespace. Sometimes, randomly generated class names are created at construct time so it turns into unattainable for elements to battle.

In case your life has been improved by CSS-in-JS, then keep it up utilizing it. Nevertheless, it’s value understanding the advantages of the CSS cascade slightly than working towards it on each venture. For instance, you possibly can set default fonts, colours, sizes, tables and type fields which might be universally utilized to each aspect in a single place. There’s not often a have to declare each fashion in each element.

10. Simplify Selectors

Even probably the most complicated CSS selectors take milliseconds to parse, however decreasing complexity will scale back file sizes and assist browser parsing. Do you actually need this kind of selector?!

physique > important.principal > part.first h2:nth-of-type(odd) + p::first-line > a[href$=”.pdf”]

Once more, be cautious of deep nesting in preprocessors reminiscent of Sass, the place complicated selectors could be inadvertently created.

11. Be Cautious of Costly Properties

Some properties are slower to render than others. For added jankiness, attempt putting field shadows on all of your parts!

*, ::earlier than, ::after
box-shadow: 5px 5px 5px rgba(zero,zero,zero,zero.5);

Browser efficiency will differ however, usually, something which causes a recalculation earlier than portray will probably be extra pricey when it comes to efficiency:

  • border-radius
  • box-shadow
  • opacity
  • rework
  • filter
  • place: fastened

12. Undertake CSS Animations

Native CSS transitions and animations will all the time be quicker than JavaScript-powered results that modify the identical properties. CSS animations won’t work in older browsers akin to IE9 and under, however these customers won’t ever know what they’re lacking.

That stated, keep away from animation for the sake of it. Delicate results can improve the consumer expertise with out adversely affecting efficiency. Extreme animations might sluggish the browser and trigger movement illness for some customers.

13. Keep away from Animating Costly Properties

Animating the size or place of a component may cause the entire web page to re-layout on each body. Performance could be improved if the animation solely impacts the compositing stage. Probably the most environment friendly animations use:

  • opacity and/or
  • rework to translate (transfer), scale or rotate a component (the unique area the component used shouldn’t be altered).

Browsers typically use the hardware-accelerated GPU to render these results. If neither are superb, think about taking the factor out of the web page movement with place: absolute so it may be animated in its personal layer.

14. Point out Which Parts Will Animate

The desire-change property permits CSS authors to point a component can be animated so the browser could make efficiency optimizations prematurely. For instance, to declare that a component could have a rework utilized:

will-change: rework;

Any variety of comma-separated properties could be outlined. Nevertheless:

  • use will-change as a final resort to repair efficiency points
  • don’t apply it to too many parts
  • give it enough time to work: that’s, don’t start animations instantly.

15. Undertake SVG Pictures

Scalable vector graphics (SVGs) are sometimes used for logos, charts, icons, and easier diagrams. Quite than outline the colour of every pixel like JPG and PNG bitmaps, an SVG defines shapes resembling strains, rectangles and circles in XML. For instance:

<svg xmlns=”” viewBox=”0 0 800 600″>
<circle cx=”400″ cy=”300″ r=”50″ stroke-width=”5″ stroke=”#f00″ fill=”#ff0″ />

Easier SVGs are smaller than equal bitmaps and may infinitely scale with out dropping definition.

An SVG might be inlined instantly in CSS code as a background picture. This may be superb for smaller, reusable icons and avoids further HTTP requests. For instance:

background: url(‘knowledge:picture/svg+xml;utf8,<svg xmlns=”” viewBox=”0 0 800 600″><circle cx=”400″ cy=”300″ r=”50″ stroke-width=”5″ stroke=”#f00″ fill=”#ff0″ /></svg>’) middle middle no-repeat;

16. Type SVGs with CSS

Extra sometimes, SVGs are embedded immediately inside an HTML doc:

<svg class=”mysvg” xmlns=”” viewBox=”0 0 800 600″>
<circle cx=”400″ cy=”300″ r=”50″ />

This provides the SVG nodes instantly into the DOM. Subsequently, all SVG styling attributes may be utilized utilizing CSS:

stroke-width: 1em;

stroke-width: 5px;
stroke: #f00;
fill: #ff0;

The quantity of embedded SVG code is lowered and the CSS types might be reused or animated as needed.

Word that utilizing an SVG inside an <img> tag or as a CSS background picture means they’re separated from the DOM, and CSS styling could have no impact.

17. Keep away from Base64 Bitmap Pictures

Normal bitmap JPGs, PNGs and GIFs might be encoded to a base64 string inside a knowledge URI. For instance:

background-image: url(‘knowledge:picture/png;base64,ABCDEFetc+and so on+and so on’);


  • base64 encoding is usually 30% bigger than its binary equal
  • the browser should parse the string earlier than it may be used
  • altering a picture invalidates the entire (cached) CSS file.

Whereas fewer HTTP requests are made, it not often offers a noticeable profit — particularly over HTTP/2 connections. Typically, keep away from inlining bitmaps until the picture is unlikely to vary typically and the ensuing base64 string is unlikely to exceed a couple of hundred characters.

18. Contemplate Essential CSS

These utilizing Google web page evaluation instruments will typically see recommendations to “inline critical CSS” or “reduce render-blocking stylesheets”. Loading a CSS file blocks rendering, so efficiency might be improved with the next steps:

  1. Extract the types used to render parts above the fold. Instruments resembling criticalCSS may also help.
  2. Add these to a <fashion> component within the HTML <head>.
  3. Load the primary CSS file asynchronously utilizing JavaScript (maybe after the web page has loaded).

The method undoubtedly improves efficiency and may benefit Progressive Net or single-page apps which have constant interfaces. Positive factors could also be much less clear for different websites/apps:

  • It’s unattainable to determine the “fold”, and it modifications on each system.
  • Most websites have differing web page layouts. Every one might require totally different essential CSS, so a construct device turns into important.
  • Dynamic, JavaScript-driven occasions might make above-the-fold modifications that aren’t recognized by important CSS instruments.
  • The method principally advantages the consumer’s first web page load. CSS is cached for subsequent pages so further inlined types will improve web page weight.

That stated, Google will love your website and push it to #1 for each search time period. (web optimization “experts” can quote me on that. Everybody else will realize it’s nonsense.)

19. Think about Progressive Rendering

Relatively than utilizing a single site-wide CSS file, progressive rendering is a way that defines particular person stylesheets for separate elements. Every is loaded instantly earlier than a element is referenced within the HTML:


<!– core types used throughout elements –>
<hyperlink rel=’stylesheet’ href=’base.css’ />


<!– header element –>
<hyperlink rel=’stylesheet’ href=’header.css’ />

<!– main content material –>
<hyperlink rel=’stylesheet’ href=’content material.css’ />

<!– type styling –>
<hyperlink rel=’stylesheet’ href=’type.css’ />


<!– header element –>
<hyperlink rel=’stylesheet’ href=’footer.css’ />


Every <hyperlink> nonetheless blocks rendering, however for a shorter time, as a result of the file is smaller. The web page is usable sooner, as a result of every element renders in sequence; the highest of the web page might be seen whereas remaining content material masses.

The method works in Firefox, Edge and IE. Chrome and Safari “optimize” the expertise by loading all CSS information and displaying a white display whereas that happens — however that’s no worse than loading every within the <head>.

Progressive rendering may benefit giant websites the place particular person pages are constructed from a number of totally different elements.

20. Study to Love CSS

An important tip: perceive your stylesheets!

Including huge portions of CSS from StackOverflow or BootStrap might produce fast outcomes, however it’s going to additionally bloat your codebase with unused junk. Additional customization turns into frustratingly troublesome, and the appliance won’t ever be environment friendly.

CSS is straightforward to study however troublesome to grasp. You’ll be able to’t keep away from the know-how if you wish to create efficient client-side code. Somewhat information of CSS fundamentals can revolutionize your workflow, improve your apps, and noticeably enhance efficiency.

Have I missed your favourite CSS efficiency tip?