부트스트랩(bootstrap)

h1

button

grid

margin
m - for classes that set margin
t - for classes that set margin-top or padding-top
x - for classes that set both -left and -right
y - for classes that set both -top and -bottom
auto - for classes that set the margin to auto

image
.img-fluid. This applies max-width: 100%; and height: auto; to the image so that it scales with the parent width.

스마트폰/패드 사용시 반응형 고려할 때 이미지도 신경써야한다.