Explain concepts important to validation of email addresses with regular expressions. Applies to most languages/tools that uses regular expressions.
Consider email address RFCs:
Look at HTML5 form input element (with type=email) regex pattern:
Learn how to create flexible layouts using relative units in CSS. New relative units added in CSS3 include vw(viewports width), vh(viewports height), vmin(minimum of viewports height and width), and vmax (maximum of viewports height and width).