<head> contains invalid HTML elements
This means that the URL in question contains a <noscript> tag in the <head>, which includes invalid HTML elements.
Why is this important?
The <noscript> tag defines an alternate content for users that have disabled scripts in their browser or have a browser that doesn't support script.
Whilst it can be used in both the <head> and the <body>, when used inside the <head>, it must contain only <link>, <style>, and <meta> elements.
The inclusion of other HTML elements can be problematic for search engines crawlers that do not render JavaScript (i.e. most crawlers, most of the time), as the presence of other elements breaks the <head>, which may cause important tags (e.g. meta robots) to be missed.
What does the Hint check?
This Hint will trigger for any internal URL that contains <noscript> in the <head>, with an invalid element (i.e. anything other than <link>, <style>, and <meta> elements).
Examples that trigger this Hint
This Hint will trigger for any URL that has an invalid element, within <noscript> in the <head>. For example:
How do you resolve this issue?
This Hint is marked 'Critical' as it represents a fundamentally breaking issue, which may have a serious adverse impact upon organic search traffic. It is strongly recommended that Critical issues are dealt with as a matter of high priority.
You will first need to establish what the <noscript> is and what it is doing. It may be possible to simply move the <noscript> out of the <head>, and into the <body>, where such elements are valid.
All invalid elements should be removed from the <noscript>, if it is to remain in the <head>.
Google's documentation on the matter is as follows:
"We strongly recommend that you don't use these invalid elements in the <head>, but if you must, place these invalid elements after the ones you want Google to see. Once Google detects one of these invalid elements, it assumes the end of the <head> and stops reading any further elements in the <head>."