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])
More Related Contents:
- Is there a CSS parent selector?
- Complex CSS selector for parent of active child [duplicate]
- Select every Nth element in CSS
- CSS3 selector :first-of-type with class name?
- CSS Selector that applies to elements with two classes
- Why isn’t it possible to combine vendor-specific pseudo-elements/classes into one rule set?
- CSS ‘>’ selector; what is it? [duplicate]
- CSS selector based on element text? [duplicate]
- Pure CSS checkbox image replacement
- Handling a colon in an element ID in a CSS selector [duplicate]
- How to write a CSS hack for IE 11? [duplicate]
- :not() selector not behaving the same between Safari and Chrome/Firefox
- CSS selector for attribute names based on a wildcard
- IE8 :nth-child and :before
- Browser Support for CSS Page Numbers
- CSS selector to select first element of a given class
- Can I use variables for selectors?
- Combine :after with :hover
- CSS: styled a checkbox to look like a button, is there a hover?
- Are CSS3 ::before and ::after pseudo elements supported by IE9 or not?
- :active css selector not working for IE8 and IE9
- CSS Selector for
- How can I zoom an HTML element in Firefox and Opera?
- How can I apply styles to multiple classes at once?
- Is it possible to select the last n items with nth-child?
- CSS selector involving pseudo class first-child and dropcap
- Multiple descendant children selector with css [duplicate]
- Targeting position:sticky elements that are currently in a ‘stuck’ state
- CSS class repetition to increase specificity
- select second child