What is the Definition of a “CSS Hack”?

By Louis Lazaris

If you’ve been writing CSS for at least a couple of years, then you’ve most certainly used a CSS hack. But if you’re relatively new to CSS, it’s possible you’ve heard the term but aren’t sure exactly what it means.

In this post, I’m going to explain what exactly the term CSS hack means and how a CSS hack is used. But first, some background to explain why I felt this post was even necessary.

Many Developers Seem to Misunderstand the Term

As many of you are aware, SitePoint recently published the results of a large CSS survey that I put together. One of the questions the survey asked was the following:

Which of the following Microsoft Browsers do you currently write or include CSS hacks for?

When I first studied the results, I seemed to have missed an oddity in the results for this question. Fortunately, David Storey, who is an engineer working on Microsoft’s newest browser, pointed it out. Of the 1,418 people who answered this question, the results went like this:

  • IE9 – 62%
  • IE10 – 61%
  • IE11 – 57%
  • Edge – 45%
  • IE8 – 35%
  • IE7 – 9%
  • IE6 – 3%
  • IE5.5 – 1%

It’s bad enough that more than 60% of developers are claiming to write CSS hacks for IE9 and IE10 – but 45% for Edge? Although there are some published hacks for Edge, they aren’t yet on the Browserhacks website, so it seems unlikely that so many people are using hacks for that browser. But the more important question is: What problems are developers running into with rendering CSS in Edge that they’re requiring hacks?

At first, I thought it might be that many of the participants are confusing hacks with browser detection via User Agent sniffing. But even that wouldn’t explain why the number is so high for Edge.

Then I realized they must have misunderstood the question completely; they think ‘writing CSS hacks for browser x’ is the same as ‘supporting browser x’. There’s really no other logical explanation, especially when you consider the high percentages for the other browsers that also shouldn’t need hacks.

So let’s define exactly what a hack is, for those who might be confused by the term.

What is a CSS Hack?

For something in your CSS file to be considered a “hack” it must apply its styles only to the browser(s) being targeted while all other browsers ignore it.

Let’s consider an example. This is a CSS hack:

* html .sidebar {
  margin-left: 5px;

The CSS in the above example (often referred to as the “star-html hack“) will target only Internet Explorer versions 6 and below. Most developers who support IE6 don’t really care about anything before IE6, so this usually works as an IE6-only hack.

The part that is the “hack”, is the asterisk followed by the “html”. This is a combination of the universal selector and the element type selector. At some point, someone discovered that these two selectors together preceding another selector work only in certain versions of IE while having no effect in other browsers. This means that the left margin on the .sidebar element defined in the above code example will apply only to IE6 or earlier. In this case, the CSS is actually valid, so you won’t get an error or warning about it (more on this later).

Here’s another example taken from the Browserhacks website, this time targeting IE11:

_:-ms-fullscreen, :root .selector {
  margin-left: 5px;

I’m not going to go into the specifics of why this is a hack (partly because I’m not entirely sure I understand it), but the above CSS will apply only to Internet Explorer version 11. Technically, Browserhacks says ‘IE11 and above’, so I’m assuming this means it will also work in Microsoft’s Edge browser, but I haven’t verified that.

The important point here is not which browsers are targeted, but that we’re all on the same page in understanding what is a CSS hack.

Are CSS Hacks Invalid CSS?

If you have hacks in your stylesheet, it’s possible that your CSS will produce warnings and/or errors if you run it through the W3C’s CSS validator. But that’s not a guarantee, nor is it a way to recognize if something is a hack.

It’s possible that your CSS can contain hacks and produce no warnings or errors. For example, if the only CSS hacks you use are targeting IE6 using the star-html hack, your stylesheets will validate just fine with no errors or warnings associated with hacks.

Also, some hacks (like the IE11 hack I discussed above) use vendor-specific code (e.g. :-ms-fullscreen). In such a case, the default settings in the validator could display your CSS with the “pass” green screen message:

W3C Validator green pass screen

But if you scroll down on the validator screen, you’ll see warnings like this:

Validator warning message for IE-specific CSS

In this case, it’s warning me because :-ms-fullscreen is considered “an unknown vendor extended pseudo-class”. If you feel more comfortable viewing this kind of CSS as an error instead of just a warning, you can adjust the validator’s settings using the “More Options” section below the validator’s input area:

More Options section on W3C Validator page

Changing the “Vendor Extensions” option to “Errors” will prevent a stylesheet from passing validation if it contains vendor prefixes or other browser-specific CSS (not necessarily hacks).

On the other hand, you might use something like this:

.example {
  margin-left: 5px\9;

The above CSS targets IE8 and below. The “hack” is the combination of the backslash and the nine (\9). Most browsers will ignore the full line, because the \9 portion makes the line invalid CSS. But, for whatever reason, Internet Explorer versions 8 and lower will still view it as valid and will apply the margin setting.

In this case, however, no matter what settings you choose for the validator, it will display an error message and the stylesheet will not pass validation:

Validator error message for IE8 CSS hack

What Techniques Are Not Hacks?

The following methods and techniques should not necessarily be categorized as CSS hacks:

!important declarations are not hacks

A line of CSS with the !important keyword appended is a different issue altogether. This is valid CSS and is not used to target a specific browser. This is not a hack, but it could be viewed as bad CSS.

Vendor prefixes are not necessarily hacks

Vendor prefixes target specific browsers, but these are not what we customarily refer to as hacks. In most cases, if you’re using vendor prefixes, then you’re also supplying valid accompanying standard code. That’s not a hack. That being said, there are some cases where you would write vendor-specific code to target a browser, thus qualifying as a hack. The above code that uses _:-ms-fullscreen is an example. Another good example is triggering hardware acceleration in WebKit browsers. But vendor prefixes, for the most part, are a separate subject. In fact, the W3C has documentation on vendor-specific code, supporting the notion that these should not, in themselves, be viewed as hacks.

High-specificity selectors are not hacks

Being really specific with your selectors to try to override something in another part of a stylesheet (e.g. body .content #sidebar p) is not a CSS hack. It’s bad CSS, but it’s not a hack.

Old syntax is not a hack

A good example of this is the complex code required for deep browser support in Flexbox. Besides the fact that this is probably unnecessary today, I would not categorize that sort of thing as a hack. It was valid code when those browsers supported it, so it might have the same effect as a hack, but I don’t think it’s the same thing.

What About Conditional Comments?

Conditional comments that allow you to write CSS or even HTML to target certain versions of Internet Explorer (or even to exclude certain versions of Internet Explorer) are a bit of a gray area. If written a certain way, they’re valid HTML, but they are “hacky”.

Back in 2008, Paul Irish popularized what we refer to as “conditional classes”, which I’m sure many of us have used. These use conditional comments to produce classes that you can use in your stylesheet to target specific versions of IE using valid CSS.

So is the use of conditional comments a “CSS hack”? I would say yes, just because they accomplish exactly the same thing intended when using a more customary CSS hack.

Should You Use CSS Hacks?

As is the case with many web development topics, the answer here is not simply yes or no. The correct answer is it depends. Most purists will say don’t use them. But it’s often not that simple. When it comes to hacks, my advice is this:

  1. Do everything in your power, within the project’s budget and time constraints, to write valid, standards-based, cross-browser CSS without using hacks.
  2. If you’ve exhausted all avenues or don’t have the time or budget to fix the problem with valid, hack-free CSS, then go ahead and use a hack.
  3. When writing your hacks, do something like what Harry Roberts recommends, so your hacks are separated and easier to refactor when your time and budget allows.
  4. Always include comments (or documentation) with the hack and try to refactor the code as soon as possible.

In Summary

If you remember nothing else from this post, remember this:

A CSS hack applies CSS in one or more specific browser versions while that same CSS will be ignored by other browsers.

That is the simple definition of a CSS hack. So just because you support Microsoft Edge in your CSS, doesn’t mean you write hacks for Edge; support is a different topic altogether.

There might be a few things in this post that not everyone agrees with, but I think most developers who understand what hacks are will agree with the above concluding summary.

If I’ve left anything out or made any errors, feel free to let me know in the comments and I’ll make any necessary corrections.

  • mbokil

    When I was forced to support IE8 in the day the 9 hack was awesome. Luckily most our customers use IE11 and Chrome now.

  • Jeroen Meeus

    Also don’t forget the way a low of people develop. They develop in chrome / firefox. and then test the other browsers. They then believe that they are adding hacks for *the other* browser. IE/Edge is often the other browser.

    • mbokil

      Pretty much everyone I know develops this way. We just don’t have enough time to build and test everything. If our customers are mostly on Chrome and IE11 then those become the target for testing. Such is capitalism. Have to move fast to stay afloat.

  • ntamas

    Putting a browser-specific class on the body or html tags sounds a much better approach to me. Sniffing is anti-pattern of course, but conditional comments will do the job for older IEs.

    • Thierry Koblentz

      In my opinion, using specific classes to target browsers is not “safe” simply because that class will add more specificity to the styling.

      For example, imagine this:

      p {margin: 10px 0;}

      .ie7 p {margin: 20px 0;}

      .paragraph-intro {margin: 40px 0;}

      In IE7, “.paragraph-intro” will not get that custom margin.

      On the other hand, the following will work fine (bonus: the property/value pairs are next to each other so it’s easier to maintain those values):

      p {
      margin: 10px 0;
      *margin: 20px 0;

      .paragraph-intro {margin: 40px 0;}

      In my opinion, such CSS filters/hacks may look ugly but they do a much better job than solutions that claim to be cleaner/neater.

      • ntamas

        In this case it does break things, I agree. But that could be easily avoided by using BEM, or some other low-specificity enforcer methodology.

        Hacks also have their drawbacks:
        – you never know how your minifier upgrade breaks your final css.
        – does your teammate knows what was your intention with a hack-like thing?(maybe it’s only a typo? let’s remove it)
        – breaks the syntax highlighting in your editor
        – you can’t be sure when the next version of a random browser will start to understand and applying them.

        Browser-specific rules are better for these, and can be easily excluded when you stop supporting legacy browsers.

        However, I’m not sure when was the last time when I needed to write different css for different browsers. :)

  • LouisLazaris

    For those interested, Thierry Koblentz had some good thoughts on Twitter on the definition of “hack”:

    As you’ll note in the discussion under the main tweet, techniques like this padding trick are often called “hacks”. But in my opinion, those are merely “tricks” that do exactly what the spec outlines. Whereas the “hacks” I’m describing in this article only target certain browsers, so I feel more comfortable using the phrase “CSS hacks” for those kinds of things.

    • Thierry Koblentz

      Hi Louis,

      I totally agree with you that the “padding trick” should *not* be called a hack. Simply because it is plain CSS working according to spec.

      I think I now better understand the distinction you make between a “hack” and a “trick”. Am I right saying that what you consider a “hack” is a technique that does not rely on “standards” while a trick leverages specs in a “non-obvious” way?

      In other words, even if “* html” and “:root body” are both used as CSS filters, the former is a hack while the latter is a trick. Do you agree with this interpretation?

      • LouisLazaris

        I don’t think I can commit to a specific definition other than what I stated in the article. I don’t think it’s necessarily an anti-standards thing, because sometimes the hack uses vendor-specific code, which, technically, is supposed to be something on a standards track.

        Like I said in the article:

        A CSS hack applies CSS in one or more specific browser versions while that same CSS will be ignored by other browsers.

        As long as the technique accomplishes that, it can be called a “hack”, so I don’t think we need to narrow it down to whether something is based on standards or not (but in most cases it will be going against standards or utilizing a rendering bug).

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        • SelenIT

          Louis, shouldn’t this definition be updated with just one extra word: “…while that same CSS will *always* be ignored by other browsers” (or, at least, something like “…while that same CSS will very unlikely work in any other browser”)?

          For example, the following CSS selector: .selector:not(*:root) {}. Currently, it’s supported only in WebKit/Blink, and is thus often listed as a “hack” for these engines. It’s also invalid CSS3 (per CSS3 Selectors spec, :not() pseudoclass can take only a simple selector as an argument, i.e. either one universal selector, one type selector, one class selector or one pseudo selector, but not their combination). But per CSS Selectors 4 spec, this selector becomes valid, so at some point other browsers can start supporting it. Should it be considered vendor-specific code at all?

          • LouisLazaris

            I think you kind of answered your own question. It’s hard to say “always” because you never know if another browser will support something in the future. A hack work in the way I described, and ideally it would never work in other browsers, but I don’t think that needs to be the case for the definition. It’s just preferable that way.

          • SelenIT

            I suppose that something that “not intended to work in other browsers” part is a good key to the answer. Using the clearly buggy behavior of one browser to do something useful (e.g. to work around its other bugs) is definitely a hack. Using vendor-prefixed code (even if this code ends up being supported by other browsers, as it happened to several -webkit-prefixed features some years ago) — probably a hack, too. But using a standard (or, at least, intended to become standard) feature that just has currently a very limited support — is it still a hack? I doubt.

  • Franklyn Roth

    Good write up. I’ve heard the phrase “this is hackey” a lot of times while I was learning front-end development in school and as an intern. Thanks for clearing up what an actual css hack is. I will spread the word.

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