Get element's source only in Selenium or Marionette. Both Selenium and Marionette have APIs to get source of the entire page but not just the source of the `WebElement` or `HTMLElement`. This utility function fills that gap.

gistfile1.py

def element_html(driver, elements):
    """Return source of a particular HTML element on the page.
    
    :param driver: webdriver or marionette object - the object you use
                   to drive your webdriver or marionette tests.
    
    :param elements: Webdriver.remote.webelement.WebElement object or
                     marionette.marionette.HTMLElement object or
                     a list of WebElement or HTMLElement objects
               
    :returns: string or list of strings of HTML element's source
    """
    
    try:
        from selenium.webdriver.remote.webelement import WebElement
    except:
        from marionette.marionette import HTMLElement

        # just make this any type/class that is irrelevant
        WebElement = str
    finally:
        if locals().get('HTMLElement', None) is None:
            # just make this any type/class that is irrelevant
            HTMLElement = str

    def get_source(element):
        return driver.execute_script("return arguments[0].innerHTML;", element)

    ret_val = None
    if isinstance(elements, list):
        ret_val = []
        for el in elements:
            ret_val.append(get_source(el))

    elif isinstance(elements, WebElement) or isinstance(elements, HTMLElement):
        if HTMLElement != str:
            elements = [elements]
        ret_val = get_source(elements)

    else:
        raise Exception("Provide a webdriver's WebElement/marionette's "
                        "HTMLElement or a list of those objects.")

    return ret_val