HTML 5 provides a clean way to declare the document encoding:
        <meta charset="UTF-8">

compared to the old way
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

Swing does not support the newer way of specifying the encoding.

Note: UTF-8 is the only valid value for charset attribute of the <meta> element.

HTML5 Spec: https://html.spec.whatwg.org/
HTML5 meta element: https://html.spec.whatwg.org/multipage/semantics.html#the-meta-element
HTML5 charset attribute of meta element: https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-charset
HTML5 pragma directives (http-equiv="content-type"): https://html.spec.whatwg.org/multipage/semantics.html#pragma-directives

Charset attribute of meta element at MDN: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta#charset