are html5 data attributes case insensitive?

HTML5 tags and attributes ARE case INsensitive.

It makes no sense to get information from browser vendors. Instead, you should get that information directly from the source (W3C standards): http://w3c.github.io/html-reference/documents.html#case-insensitivity

You can either use XHTML or HTML5. From a web page loading point of view only HTML5 makes sense currently.

In documents in the HTML syntax: • Tag names for HTML elements may be written with any mix of lowercase and uppercase letters that are a case-insensitive match for the names of the elements given in the HTML elements section of this document; that is, tag names are case-insensitive. • Attribute names for HTML elements may be written with any mix of lowercase and uppercase letters that are a case-insensitive match for the names of the attributes given in the HTML elements section of this document; that is, attribute names are case-insensitive.

You should always use lowercase characters. Even though some browser do automatically correct mistakes in the markup (that's what Chrome does for you here) it can cause errors and isn't valid HTML5. From MDN:

• the name must not start with xml, whatever case is used for these letters;
• the name must not contain any semicolon (U+003A);
• the name must not contain capital A to Z letters.

EDIT

After some more research I've found this:

All attribute names on HTML elements in HTML documents get ASCII-lowercased automatically, so the restriction on ASCII uppercase letters doesn't affect such documents.