https://daverupert.com/2017/11/happier-html5-forms/

• html5
• form
• validation
• invalid
• submit

http://alistapart.com/article/forward-thinking-form-validation

• form
• validation
• valid
• invalid
• focus
• required

http://davidwalsh.name/valid-pseudo-classes

• css3
• valid
• invalid
• required
• pseudo
• class
• validation
• html5