何时在 CSS 中使用! important 属性

考虑一下:

#div p {
color: red !important;
}
...
#div p {
color: blue;
}

我了解 !important的工作原理。在这种情况下，div 将呈现红色，因为现在它具有优先级(!important)。但我还是找不到合适的场合使用它。有没有 !important拯救世界的例子？

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最佳答案

This is the real life scenario

Imagine this scenario

  1. You have a global CSS file that sets visual aspects of your site globally.
  2. You (or others) use inline styles on elements themselves which is usually very bad practice.

In this case you could set certain styles in your global CSS file as important, thus overriding inline styles set directly on elements.

Actual real world example?

This kind of scenario usually happens when you don't have total control over your HTML. Think of solutions in SharePoint for instance. You'd like your part to be globally defined (styled), but some inline styles you can't control are present. !important makes such situations easier to deal with.

Other real life scenarios would also include some badly written jQuery plugins that also use inline styles...

I suppose you got the idea by now and can come up with some others as well.

When do you decide to use !important?

I suggest you don't use !important unless you can't do it any other way. Whenever it's possible to avoid it, avoid it. Using lots of !important styles will make maintenance a bit harder, because you break the natural cascading in your stylesheets.

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!important is somewhat like eval. It isn't a good solution to any problem, and there are very few problems that can't be solved without it.

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Overwriting the Style Attribute

Say in the example that you are unable to change the HTML source code but only provide a stylesheet. Some thoughtless person has slapped on a style directly on the element (boo!)

       div { background-color: green !important }
    <div style="background-color:red">
<p>Take that!</p>
</div>

Here, !important can override inline CSS.

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You generally use !important when you've run out of other ways to increase the specificity of a CSS selector.

So once another CSS rule has already dabbled with Ids, inheritance paths and class names, when you need to override that rule then you need to use 'important'.

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This is a real, real life scenario, because it actually happened yesterday:

Alternatives to not using !important in my answer included:

  • Hunting down in JavaScript/CSS where a certain elusive property was being applied.
  • Adding the property with JavaScript, which is little better than using !important.

So, a benefit of !important is that it sometimes saves time. If you use it very sparingly like this, it can be a useful tool.

If you're using it just because you don't understand how specificity works, you're doing it wrong.

Another use for !important is when you're writing some kind of external widget type thing, and you want to be sure that your styles will be the ones applied, see:

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I have to use !important when I need to overwrite the style of an HTML generated by some JavaScript "plugin" (like advertising, banners, and stuff) that uses the "style" attribute.

So I guess that you can use it when you don't control the CSS.

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Strictly speaking you shouldn't need to use !important if you've structured your CSS well and don't have too many degrees of specificity.

The most appropriate time to use !important is when you have one exceptional style that you want to style outside of your site's normal cascade.

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You use !important to override a css property.

For example, you have a control in ASP.NET and it renders a control with a background blue (in the HTML). You want to change it, and you don't have the source control so you attach a new CSS file and write the same selector and change the color and after it add !important.

Best practices is when you are branding / redesigning SharePoint sites, you use it a lot to override the default styles.

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This is a real-world example.

While working with GWT-Bootstrap V2, it will inject some CSS file, which will override my CSS styles. In order to make my properties to be not overridden, I used !important.

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The use of !important is very import in email creation when inline CSS is the correct answer. It is used in conjunction with @media to change the layout when viewing on different platforms. For instance the way the page looks on desktop as compare to smart phones (ie. change the link placement and size. have the whole page fit within a 480px width as apposed to 640px width.

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I'm using !important to change the style of an element on a SharePoint web part. The JavaScript code that builds the elements on the web part is buried many levels deep in the SharePoint inner-workings.

Attempting to find where the style is applied, and then attempting to modify it seems like a lot of wasted effort to me. Using the !important tag in a custom CSS file is much, much easier.

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Using !important is generally not a good idea in the code itself, but it can be useful in various overrides.

I use Firefox and a dotjs plugin which essentially can run your own custom JS or CSS code on specified websites automatically.

Here's the code for it I use on Twitter that makes the tweet input field always stay on my screen no matter how far I scroll, and for the hyperlinks to always remain the same color.

a, a * {
color: rgb(34, 136, 85) !important;
}


.count-inner {
color: white !important;
}


.timeline-tweet-box {
z-index: 99 !important;
position: fixed !important;
left: 5% !important;
}

Since, thankfully, Twitter developers don't use !important properties much, I can use it to guarantee that the specified styles will be definitely overridden, because without !important they were not overridden sometimes. It really came in handy for me there.

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I am planning to use !important for a third-party widget meant to be embedded in a large number of websites out of my control.

I reached the conclusion !important is the only solution to protect the widget's stylesheet from the host stylesheet (apart from iframe and inline styles, which are equally bad). For instance, WordPress uses:

#left-area ul {
list-style-type: disc;
padding: 0 0 23px 16px;
line-height: 26px;
}

This rule threathens to override any UL in my widget because id's have strong specificity. In that case, systematic use of !important seems to be one of the few solutions.


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