Please recommend a css library such as bootstrap.

In my case, I just customize the bootstrap, but I recommend the bootstrap-live-customizer. Or you can just use the free theme provided by bootswatch. As for neatness, uikit and material ui sites are generally useful.

http://cssdb.co/

http://bootswatch.com Bootstrap theme

http://bootstrap-live-customizer.com/ (Bootstrap theme + customizer)

http://www.material-ui.com/

https://getmdl.io/index.html

http://picnicss.com/

http://getuikit.com/

http://materializecss.com/getting-started.html

http://purecss.io/

http://semantic-ui.com/

http://picnicss.com/

https://milligram.github.io/

http://www.minimamente.com/example/magic_animations/

http://elrumordelaluz.github.io/csshake/

http://daneden.github.io/animate.css/

http://bouncejs.com/

http://ianlunn.github.io/Hover/

http://projects.lukehaas.me/css-loaders/

http://tobiasahlin.com/spinkit/

http://tobiasahlin.com/typesource/

https://www.google.com/fonts

http://flexboxgrid.com/

https://codyhouse.co/gem/product-tour/

Metallic Components from Google are also recommended. It supports iOS, Android, and web. https://material.io/components/