Similar to https://github.com/python-visualization/folium/blob/master/folium/plugins/float_image.py , but more generic. Seemed odd that such a thing didn't already exist.

float_div.py

import branca
import jinja2

class FloatDiv(branca.element.MacroElement):
    """Adds a floating div in HTML canvas on top of the map."""
    _template = jinja2.Template("""
            {% macro header(this,kwargs) %}
                <style>
                    #{{this.get_name()}} {
                        position:absolute;
                        top:{{this.top}}%;
                        left:{{this.left}}%;
                        }
                </style>
            {% endmacro %}
            {% macro html(this,kwargs) %}
            <div id="{{this.get_name()}}" alt="float_div" style="z-index: 999999">
              {{this.content}}
            </div
            {% endmacro %}
            """)

    def __init__(self, content, top=10, left=0):
        super(FloatDiv, self).__init__()
        self._name = 'FloatDiv'
        self.content = content
        self.top = top
        self.left = left