Page Title modified by JavaScript
This means that the URL in question has a page title that is different in the response HTML of the page, when compared with the rendered HTML.
Why is this important?
Google clearly state in their documentation that it is fine to use JavaScript to set or change the page title:
However, this doesn't mean that this is the optimal way to deliver page content to search engines.
The problem is that rendering webpages is a resource intensive task, and it takes significantly longer than simply grabbing source (response) HTML content.
This is why Google essentially crawl URLs in a two-stage process: their 'first look' is of the HTML response, then they render the page and have a 'second look' at the rendered HTML, then they update the index based on what they found in the rendered HTML.
As a result, URLs can and do enter the index initially based on this 'first look' (i.e. the HTML response) and there will be an indeterminate amount of time between this and the 'second look' (i.e. the rendered HTML).
So it is important that the HTML response contains all of the core elements as you want them to be included in the index, including the page title.
If the page title is being modified by JavaScript, then this implies that the 'correct' title is the one in the rendered HTML. However Google may index the page with the title found in the HTML response, and it may take some time before they are able to render the page and update the title.
This becomes particularly important if the page content (and page title) is changing all the time, you can easily end up in a situation where Google have the 'old' version indexed.
What does the Hint check?
This Hint will trigger for any internal URL that contains a different page title in the response and rendered HTML.
Examples that trigger this Hint
This Hint will trigger for any URL that has a <title> element in the <head> of the response HTML, which has a different value in the rendered HTML.
For example, if the response HTML looked like this:
And the rendered HTML looked like this:
How do you resolve this issue?
In general, it is not a good idea to rely on JavaScript for the core on-page elements, otherwise you may find that pages are getting indexed with the wrong data, or taking a lot longer to get indexed. Additionally, Google may not always be able to render pages correctly, or it may not be able to execute JavaScript at all.
The page title is one of the most important on-page SEO elements - you really want to have this available and fully optimised for Google to discover on their first look of the page.
To resolve this issue, you would need to work with the development team to ensure that the correct page title is present in the HTML response, and is not then changed by JavaScript during rendering.