_getAttributeValue(domNode, attribute) {
        while (domNode && domNode !== this._rootDomNode) {
            const line = domNode.getAttribute(attribute);
            if (line !== null) {
                return line;
            }
            domNode = domNode.parentElement;
        }
        return;
    }
Given the dom node, finds if it or its parent sequence contains the given attribute.
@returns the attribute value or undefined.