Code points

def Lean.Html.isControl (c : Char) : Bool

Whether c is a control code point.

def Lean.Html.isAsciiWhitespace (c : Char) : Bool

Whether c is ASCII whitespace. Differs from Char.isWhitespace by also including FF U+000C.

def Lean.Html.isNonCharacter (c : Char) : Bool

Whether c is a noncharacter.

Tag names

def Lean.Html.isVoidElement (tagName : String) : Bool

Whether tagName (compared case-insensitively) names a void element.

Void elements are those that cannot have any child nodes. These only have a start tag; end tags must not be specified. See https://html.spec.whatwg.org/dev/syntax.html#void-elements.