On certain audited pages, `totalBodyElements` are different from `MainDocumentContent` body nodes count

FAQ

[X] Yes, my issue is not about variability or throttling.
[X] Yes, my issue is not about a specific accessibility audit (file with axe-core instead).

URL

https://www.spie.com/en/about-us/spie-around-world

What happened?

Through a plugin, I want to count SVG child elements. To do this, I was going to use MainDocumentContent, but on the example page, the code is incomplete. When I count the number of elements in body, I get a big difference with the DOMStats data (which is good). In my tests, I have : allBodyNodesFromMainDocumentContent: 1542 artifacts.DOMStats.totalBodyElements: 4651 I'm using JSDOM to browse MainDocumentContent, but it's not the JSDOM that's causing the problem - I've checked the contents of MainDocumentContent. Here's my code:

const totalBodyElements = artifacts.DOMStats.totalBodyElements
  const MainDocumentContent = artifacts.MainDocumentContent
  const dom = new JSDOM(MainDocumentContent)
  const allBodyNodesFromMainDocumentContent = dom.window.document
    .getElementsByTagName('body')[0]
    .querySelectorAll('*').length
  const svgContentBodyNodesFromMainDocumentContent = dom.window.document
    .getElementsByTagName('body')[0]
    .querySelectorAll('svg *').length

What did you expect?

I expect allBodyNodesFromMainDocumentContent === artifacts.DOMStats.totalBodyElements

What have you tried?

I tried to find another gatherers that would contain the complete HTML code of the page, but I couldn't find anything other than MainDocumentContent.

How were you running Lighthouse?

node

Lighthouse Version

11.7.0

Chrome Version

linux-121.0.6167.85/chrome-linux64/ (from puppeteer)

Node Version

18.19.1

OS

Linux

Relevant log output

totalBodyElements 4651
allNodesFromMainDocumentContent 1565
allBodyNodesFromMainDocumentContent 1542
svgContentNodesFromMainDocumentContent 98
svgContentBodyNodesFromMainDocumentContent 98

GoogleChrome / lighthouse