<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.
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>."