As a domain expert in web development, I can tell you that CSS selectors are generally case-insensitive; this includes class and ID selectors. However, it's important to note that HTML class names are case-sensitive when it comes to how they are defined in the HTML document. If your CSS selector does not match the case of the class name in the HTML, it will not apply the styles. For instance, if you define a class in HTML as `

CSS selectors are generally case-insensitive; this includes class and ID selectors. But HTML class names are case-sensitive (see the attribute definition), and that's causing a mismatch in your second example. ... If the document type defined class names as case-insensitive, then you would have a match regardless.read more >>