Are property values in CSS case-sensitive?
The spec for CSS 2 says:
CSS syntax is case-insensitive within the ASCII range (i.e., [a-z] and [A-Z] are equivalent), except for parts that are not under the control of CSS. For example, the case-sensitivity of values of the HTML attributes "id" and "class", of font names, and of URIs lies outside the scope of this specification. Note in particular that element names are case-insensitive in HTML, but case-sensitive in XML.
... which makes quite sense: CSS itself accepts both background-image and BACKGROUND-IMAGE but it has no way to know whether your web server considers LOGO.PNG and logo.png as identical or different resources.
(I've been unable to find the equivalent document for CSS3)
(updating @ÁlvaroG.Vicario answer and comments, and complementing this answer... This is a Wiki, please edit to enhance)
Example: for CSS3 (and HTML5) there are new explicit rules, as "font-face property must be case-insensitive".[2]
Context
W3C interoperating standards, mainly XML, HTML, CSV and CSS.
CSS general rules
CSS2 (a W3C standard of 2008) fixed basic conventions about "Characters and case", and CSS3 (a W3C standard for 2015) added something more.
By default "all CSS syntax is case-insensitive (...)" [1]
There are exceptions, "(...) except for parts that are not under the control of CSS"[1]
2.1. element names are case-sensitive in HTML5 (?) and XML, but case-insensitive in HTML4.
2.2. identifiers (including element names, classes, and IDs in selectors) are case-sensitive. HTML attributes id and class, of font names, and of URIs lies outside the scope of the CSS specification.
....
The
Case matrix
Exceptions and specific (explicited in a reference) rules. "YES" indicate that value is case-sensitive.
Property values:
CSS property | Case-sens. | Reference and notes ------------------|------------|-------------------- %colorVals | NO | [3] font-family | NO | [2] %url | YES | ... content | YES | ... ---------------------------------------------------- %colorVals = color, background, etc. %url = background-image, etc. that use `url()`, see [7] and notes.
Selector values:
CSS selector | Case-sens. | Reference and notes ------------------|------------|-------------------- id | YES |... element | YES/NO | ... YES for XML... class name | YES | [5] (`~ i` operator) | NO | [6] ---------------------------------------------------- YES/NO = depends on the document language (see ref. and notes).
REFERENCES:
[1] W3C/CSS2/syndata, sec. 4.1.3 Characters and case.
[2] W3C/CSS3-fonts, sec. 5.1 Case sensitivity of font family names
[3] W3C/CSS3-color, sec. 4.1. Basic color keywords
[4] W3C/CSS3-values, sec. 3.1. Pre-defined Keywords
[5] W3C/Selectors, sec. 3. Case sensitivity
[6] W3C/Selectors4, sec. 6.3. Case-sensitivity
[7] RFC 3986 and URL syntax illustration at Wikipedia.
Quotations and notes
Typical URLs starts with domain, that is case insensitive, but after it (path, query or fragment syntatical components), is case sensitive. See [7].
"User agents must match these names case insensitively". [2]