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Editorial Team
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Asked: 2026-05-25T20:10:51+00:00

I was recently updating a CMS site and a tab-navigation plugin had inserted the

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I was recently updating a CMS site and a tab-navigation plugin had inserted the following markup:

<li id="News_tab">...

I’ve always written my CSS selectors in lowercase so when I tried to style this with #news_tab, it wouldn’t apply, but #News_tab worked.

After all these years I’m surprised that I haven’t run into this before, so I’ve always been under the impression that CSS was case-insensitive. Has CSS always been case-sensitive and I just haven’t noticed thanks to my consistent code style?

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1 Answer

  1. Editorial Team

    CSS itself is case insensitive, but selectors from HTML (class and id) are case sensitive:

    CSS recommendation on case sensitivity

    HTML recommendation, id attribute (note the [CS])

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