findElement(...args) /* ... */{
		const res = this.findElements(...args);
		return res.length ? res[0] : null;
	}
Return the first DOM node within this Map which matches the given search
parameters. This function is essentially a convenience wrapper around
`findElements()` which only returns the first found node.

This function is sensitive to the amount of arguments passed to it;
if only one argument is specified, it is used as selector-expression
as-is. When two arguments are passed, the first argument is treated
as attribute name, the second one as attribute value to match.

As an example, `map.findElement('input')` would find the first `<input>`
node while `map.findElement('type', 'text')` would find the first DOM
node with a `type="text"` attribute.

@param {string} selector_or_attrname
If invoked with only one parameter, this argument is a `querySelector()`
compatible selector expression. If invoked with two parameters, this
argument is the attribute name to filter for.

@param {string} [attrvalue]
In case the function is invoked with two parameters, this argument
specifies the attribute value to match.

@throws {InternalError}
Throws an `InternalError` if more than two function parameters are
passed.

@returns {Node|null}
Returns the first found DOM node or `null` if no element matched.