[dangrossman]

Works authored in CSS, HTML and JavaScript are all protected by copyright. Lawsuits for code theft, including in webpages, happen all the time. Ask any web host, they have to take down pages after getting DMCA copyright infringement notices about stolen HTML all the time.

Grid layouts, pinning images, etc. are ideas. Ideas are not protected. The concrete implementations of them are. You can clone Pinterest, but you can't do it by stealing their JavaScript and CSS files. You have to write your own.