<h1> modified by JavaScript
This means that the URL in question has an <h1> that is different in the response HTML of the page, when compared with the rendered HTML.
Why is this important?
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 <h1> is being modified by JavaScript, then this implies that the 'correct' <h1> is the one in the rendered HTML. However Google may index the page with the <h1> found in the HTML response, and it may take some time before they are able to render the page and update the <h1>.
This becomes particularly important if the page content (and <h1>) 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 <h1> in the response and rendered HTML.
Examples that trigger this Hint
This Hint will trigger for any URL that has a <h1> in 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 <h1> is an important on-page SEO element - 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 <h1> is present in the HTML response, and is not then changed by JavaScript during rendering.
Further Reading
- How JavaScript Rendering Affects Google Indexing
- Understand the JavaScript SEO basics
- Comparing response and rendered HTML with Sitebulb
