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Webmaster Forum / HTML, CSS, Scripts / CSS / August 2006Calculating CSS Selector Specificity
The CSS 2.1 Specification describes how to sort a list of selectors in order of specificity, but it doesn't provide a method to calculate the specificity of a single selector in isolation. I've devised a method to do this, which I describe in the following article: http://calculating-css-selector-specificity.blogspot.com/ Comments and/or criticisms welcome. > The CSS 2.1 Specification describes how to sort a list of selectors in > order of specificity, but it doesn't provide a method to calculate the > specificity of a single selector in isolation. Of course they do. www.w3.org/TR/CSS2/cascade.html#specificity > I've devised a method to do this, which I describe in the following > article: > > http://calculating-css-selector-specificity.blogspot.com/ In which you wrote, "A method currently popular is to simply concatenate the numbers a, b, c and d into a single decimal number Selector Specificity Specificity Value p#title.red 0, 1, 1, 1 0111 = 111 But this method fails if one of b, c, or d is greater than 9." This demonstrates that you overlooked their examples, in which they did exactly this--concatenating the numbers, treating them as digits base 10--and the part where they wrote "in a number system with a large base", which deals head-on with the case where any of the numbers is greater than 9. In other words, this isn't a "method currently popular". It's the way the computation was defined. > But this method fails if one of b, c, or d is greater than 9." > [quoted text clipped - 4 lines] > greater than 9. In other words, this isn't a "method currently popular". > It's the way the computation was defined. Harlan, you are refering to the CSS 2 specification. The CSS 2.1 Specification switched to defining the specificity of a selector as comma-delimited quad of integers for the precise reason I give. Follow the linke below for more information: http://meyerweb.com/eric/css/link-specificity.html > > But this method fails if one of b, c, or d is greater than 9." >> This demonstrates that you overlooked their examples, in which they did [quoted text clipped - 9 lines] > selector as comma-delimited quad of integers for the precise reason I > give. Hmm, don't know why my link wasn't updated. It doesn't matter, though. What you wrote is what 2.0 had, and I figure they dropped it from 2.1 because while it was meant to help explain it, it probably just caused more confusion, and for no purpose, because as has been pointed out to you, an absolute number doesn't mean anything. How do you compare numbers you see on paper? How do you alphabetize words? By exactly the same process used in the spec for the comma-separated numbers. The only difference between the comma-separated convention and the rendering of a number to some base is the presence or absence of the commas! Conceptualizing the specificity as an absolute number rather than as a set of values adds nothing. > The CSS 2.1 Specification describes how to sort a list of selectors in > order of specificity, but it doesn't provide a method to calculate the > specificity of a single selector in isolation. Yes it does, although it doesn't exactly result in a "number" in the strictest mathematical or physical sense, considering its base might as well be infinity. And why is such a calculation important? Why would anyone need to know how specific a selector is unless they need to know whether it "wins" over another selector? That's kind of like trying to measure the "absolute voltage" of something -- voltage is always measured as a difference in potential between two points. -- Vid the Kid > Yes it does, although it doesn't exactly result in a "number" in the > strictest mathematical or physical sense, considering its base might as > well be infinity. You're right, it does. But my point is precisely the fact that the definition doesn't result in a number. > And why is such a calculation important? Why would > anyone need to know how specific a selector is unless they need to know > whether it "wins" over another selector? I don't know if it's important or not. All I'm saying is: here's a way to do it if you think you'll find it useful. >That's kind of like trying to > measure the "absolute voltage" of something -- voltage is always > measured as a difference in potential between two points. Voltage and other physical units may be relative but specificity isn't. My article demonstrates this precise fact ! viz. Specificity is more like the abstract concept of number, you can refer to the absolute specificity of a selector in the same sense that you can refer to the absolute size of a number. >The CSS 2.1 Specification describes how to sort a list of selectors in >order of specificity, but it doesn't provide a method to calculate the [quoted text clipped - 6 lines] > >Comments and/or criticisms welcome. What do you plan to do with these numbers when you have calculated them? The "specificity of a single selector in isolation" appears to be a meaningless concept. A single selector in isolation will always be applied.  SignatureStephen Poley http://www.xs4all.nl/~sbpoley/webmatters/ > What do you plan to do with these numbers when you have calculated them? I don't know. It's just something I'm placing in the public domain which other people might find useful. > The "specificity of a single selector in isolation" appears to be a > meaningless concept. A single selector in isolation will always be > applied. It's not a meaningless concept because semantically it is identical to the comma-delimited quad notation. Whether it's a useful concept or not, I don't know. The posters so far in this thread (unanimously) do not seem to think it's useful concept but maybe other people might. >> What do you plan to do with these numbers when you have calculated them? > > I don't know. > > It's just something I'm placing in the public domain which other people > might find useful. W3 placed it in the public domain years ago. |
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