Id and other attribute to the tree's html elements

I need to add an id –attribute (or ‘rel’ -attribute) to the tree’s html elements. Id’s are needed for the Selenium test purposes. How I’m able to add ids? Functions ‘set_id’, set_userdata, and set_attribute sounds like a solution, but those values doesn’t flow into the html element as attributes as I expected.
I would be much appreciated if there any solution for this?

Unfortunately such feature is not available.

By the way, selenium can use Xpath or css locators, so it possible to located the elements without assigning some fixed IDs to them.

We have a quite large and partly old system, so xpath and css selectors slows down selenium and makes test a little tricky to write. IDs’ would make writing a new tests lots of easier.