When I clicked a mailto: link in a web page the following page appeared. How would I make it open in Gmail instead? I am using Windows 7.

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Sign up to join this communityIf you are using the latest version of Chrome:
Go to Gmail, and you should see an icon in the address bar that looks like two overlapping diamonds (see the Chrome support link here). If you click the icon and select "Use Gmail," Gmail will be used to handle your mailto: links in the future.
If you do not see the two diamonds, you may need to double-check that handlers are enabled. You can enable them by going to Chrome settings --> Show advanced settings... --> Privacy section, Content settings... --> and look under handlers to make sure you have "Allow sites to ask to become default handlers for protocols" selected (see screenshot below).
Working as of April, 2019. Chrome 73.
Let Gmail handle Mailto links:
Open Chrome -> Settings




Select Chrome as default email client
Done!
Click on a mailto: link, Gmail will open a new email window.
@LTR - Thank you for reporting old answer was no longer working.
Credit source: How to Set Gmail as Your Browser's Default Email Client for Mailto Links
So based on your tags Google Chrome appears to be your web browser, go to the Google Webstore and download Send From Gmail (By Google) which will make GMail your default email application. https://chrome.google.com/webstore/detail/send-from-gmail-by-google/pgphcomnlaojlmmcjmiddhdapjpbgeoc
For Firefox users, go to Tools in the Menu Bar, then Options, the Applications tab, then look for the Mailto: and change the dropdown menu to "Use Gmail"
You can also search "file type" in the windows search box, and select change the file type associated with a file extension. Then scroll down to MAILTO. Double click on it and select chrome.
You can just create a link like the following and it will attempt to open Gmail and create a new message. You can specify "to" and "subject" and even the "message"
<a href="https://mail.google.com/mail/u/0/?view=cm&fs=1&to=toemail@email.com&su=hello world" target="_blank">
I use Windows 7 and after sometime it stopped to work and handlers were not shown in Chrome settings (version 84.0).
In my case I used the next steps:
chrome://settings/handlers.Gmail Notifier is a better solution for all browsers.
Here is answer from Google:
http://support.google.com/mail/bin/answer.py?hl=en&answer=10966