Command "require-dev" is not defined

[stillatmylinux]

I already had composer installed, but when running

composer require-dev micheh/w3c-validator:0.*

I kept getting this error message

[Symfony\Component\Console\Exception\CommandNotFoundException]  
Command "require-dev" is not defined. 

Fixed it by running

composer clearcache
composer install

[micheh]

Hello

Composer changed the command from require-dev to require --dev, therefore you should be able to use composer require --dev micheh/w3c-validator.