html - Are property values in CSS case-sensitive?

Question

I have observed that some CSS properties, like font-family declared with quotation marks, perhaps are case-sensitive, but all other are not... But how web-browsers and "HTML renderers" MUST interpret? Is the same in any CSS context (XML, SVG, etc.) and all other applications? What the standards say about?

Example: Adobe InDesign exported both, font-family:'Optima Bold' and font-family:'optima bold'. Can I "normalize to lower case" (ex. to merge similar classes)?

NOTES

References are incomplete and in conflict:

• sitepoint.com/font-family say "Note that font family names may be case sensitive on some operating systems"... It is valid for XHTML, it is updated with HTML5? font-family is really the unique case-sensitive value?

• Is it necessary to use lowercase for every Element and attribute , properties in css and xhtml ? say indirectly "... use lowercase for every properties...", and answers not negate it.

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Comparing with this question/answers, the point here, perhaps, can be translated to some (personal) objective considerations:

1. There are a (objective!) normative (W3C spec of CSS2, CSS3, XHTML1, or HTML5) source for this answer?

2. "Standard font-family unique names" can not be case-sensitive (otherwise cease to be standard)... So, the only justifiable (by sensible arguments) properties to be case-sensitive are:

   2.1. X values at url(X), see background, etc. properties;

   2.2. content values, example;

   2.3. ... more ?? ...

Answer 1

(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]

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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.

1. By default "all CSS syntax is case-insensitive (...)" [1]

2. 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.

3. ....

4. 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.

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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]