def transform_content(tags, content_transformer):
    """
    Transform content in all `tags` using result of `content_transformer(tag)`
    call.

    Args:
        tags (obj/list): HTMLElement instance, or list of HTMLElement
                         instances.
        content_transformer (function): Function which is called as
                                        ``content_transformer(tag)``.
    """
    if type(tags) not in [tuple, list]:
        tags = [tags]

    for tag in tags:
        new_child = dhtmlparser.HTMLElement(content_transformer(tag))

        # don't forget to add parent if the list is double-linked
        if hasattr(tag, "parent"):
            new_child.parent = tag

        tag.childs = [new_child]