lxml parse doesn't recognize Indexing

I am trying to scrape and parse some HMLT with Python and extract Italian fiscal code 'VTLLCU86S03I348V'.

If I inspect the page and copy the full xPath of that object I get no results and I do not understand why. I tried with normal xpath like '//div/p/a' for example and it worked. Things become messy when the xpath contains indexing like 'div[3]/div' even though it sounds weird to me as well. I have also noticed that the inspect xpath contains just one '/' at the beginning even though I think that two are required (righ?).

Hereunder a minimal reproducible example. What am I missing?

from lxml import etree
import requests

# GET 
page = requests.get('https://notariato.it/it/notary/luca-vitale/')

# PARSING
parser = etree.HTMLParser()
tree = etree.parse(StringIO(page.text), parser)

nome_raw = tree.xpath('//html/body/div[3]/section[2]/div/div[1]/div/section/div/div/div/div[6]/div/h4')

for i in range(len(nome_raw)):
    print(nome_raw[i].text)

Thank you!

Xpaths starting with a single / are absolute ones whereas // are relative.
Using indexed elements in xpath makes it prone to fail if the page structure changes.

Giving the "label" of the element as a reference makes the xpath more readable (the rest of the code is the same)

 nome_raw = tree.xpath('//div[preceding-sibling::div/div/p[. = "Codice fiscale"]]/div/h4')
 for i in range(len(nome_raw)):
     print(nome_raw[i].text) 
VTLLCU86S03I348V

Regarding why the indexed xpath didn't work I did some tests with xmllint and it found the element

xmllint --html --recover --xpath '/html/body/div[2]/section[2]/div/div[1]/div/section/div/div/div/div[6]/div/h4' aaa.html 2>/dev/null
<h4 class="elementor-heading-title elementor-size-default">VTLLCU86S03I348V</h4>

Both lxml and xmllint are based on libxml2 but found a version difference between both

libxml2 (lxml) : 2.9.9
libxml2 (xmllint): 2.9.14

>>> from lxml import etree
>>> print ("libxml used:      ", etree.LIBXML_VERSION)
libxml used:       (2, 9, 9)
>>> print ("libxml compiled:  ", etree.LIBXML_COMPILED_VERSION)
libxml compiled:   (2, 9, 9)

From lxml docs

> HTML parsing is similarly simple. The parsers have a recover keyword argument that the HTMLParser sets by default. It lets libxml2 try its best to return a valid HTML tree with all content it can manage to parse. It will not raise an exception on parser errors. You should use libxml2 version 2.6.21 or newer to take advantage of this feature.

Not sure why it fails to parse this BROKEN html (xmllintthrew lots of errors) but a simple test shows that finds less elements than expected. Should find 6 divs

divs = tree.xpath('/html/body/div[3]/section[2]/div/div[1]/div/section/div/div/div/div')

for i in range(len(divs)):
    print(i, divs[i].tag)

Result

0 div
1 div
2 div
3 div