There is no </head>, </body> or </html> tag if you view the source on https://avenue1.nop-templates.com or https://avenue2.nop-templates.com in any desktop browser (by opening the page, right click-ing and select "View source").
After viewing the source on some of the demo sites for your themes I can see that Voyage, Pacific, Emporium and Uptown render correctly with the closing elements but Avenue, Venture, Pavilion, Prisma, Earth and Brooklyn do not.
"There is no </head>, </body> or </html> tag if you view the source on https://avenue1.nop-templates.com or https://avenue2.nop-templates.com"
That's a false alert from the browser inspector due to source code minification - the html markup is minified on the Avenue demos.
I can't tell why nopCommerce minification behaves like that in browser inspectors but that has nothing to do with the theme, it's in the framework (nopCommerce) which we don't have control over. If you inspect the nopCommerce demo you will see exactly the same thing (unclosed html, head and body tags). In fact you will see the same on every nopCommerce-based website that has minification enabled. But anyway, with or without HTML minification, the <html> &<head> & <body> tags validate normally.
"Compare Avenue and Voyage demos"
You see a difference only because Avenue has HTML minification enabled, and Voyage has it disabled. If they both have it enabled you will see no difference, and if they both have it disabled you will see no difference either.
That's the settings. It's the "HTML minification" we are talking about. Don't forget that after you disable it and click "Save", you also have to restart the application from the admin, otherwise the change will not apply.