Viewport

The viewport defines the visible area of the device the site is accessed from.

Add a viewport in <head> as:

<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0" />

Layout

3 ways:

Float layouts (old, dismiss)
Flexbox (1-dimensional rows, for simple components layout)
CSS grid (2-dimensional grid, for page layouts and complex components)

Flexbox

CSS grid

grid containers

    display: grid;
    /* number and size of columns and rows */
    grid-template-columns: 3fr auto 1fr 2fr; /* 2 fixed columns, 2 responsive columns. `fr` for fractional */
    grid-template-rows: 300px 200px ; /* sets rows heights */
    /* gaps / gutters */
    column-gap: 30px;
    row-gap: 50px;
    /* alignment */
    justify-items: stretch | start | center | end;
    align-items: stretch | start | center | end;

repeat function

    grid-template-columns: repeat(16, 1fr); /* for 16 columns, each 1fr */

align within containers

    /* cell alignment */
    justify-content: center | space-between;
    align-content: center;
    /* items alignment within cells */
    align-items: center | stretch | start | end; /* stretch is default */
    justify-items: center; /* stretch is default */

grid items

/* to place a grid item into a specific cell or group of cells */
.el-1 {
    grid-column: <start line> / <end line> | span <number>;
    grid-row: <start line> / <end line> | span <number>;
}

.el-1 {
    grid column: 1 / -1; /* -1 to span the rest of the space, eg full row here */
    grid row: 2 / 3;
}

align within grid items

.el-1 {
    align-self: center;
    justify-self: center;
}

CSS

media queries

Use @media to change properties based on viewport size.

Mobile first means changing properties if viewport is larger than mobile:

/* For mobile phones: */
[class*="col-"] {
  width: 100%;
}

@media only screen and (min-width: 600px) {
  /* For tablets: */
  .col-s-1 {width: 8.33%;}
  .col-s-2 {width: 16.66%;}
  .col-s-3 {width: 25%;}
  .col-s-4 {width: 33.33%;}
  .col-s-5 {width: 41.66%;}
  .col-s-6 {width: 50%;}
  .col-s-7 {width: 58.33%;}
  .col-s-8 {width: 66.66%;}
  .col-s-9 {width: 75%;}
  .col-s-10 {width: 83.33%;}
  .col-s-11 {width: 91.66%;}
  .col-s-12 {width: 100%;}
}

@media only screen and (min-width: 768px) {
  /* For desktop: */
  .col-1 {width: 8.33%;}
  .col-2 {width: 16.66%;}
  .col-3 {width: 25%;}
  .col-4 {width: 33.33%;}
  .col-5 {width: 41.66%;}
  .col-6 {width: 50%;}
  .col-7 {width: 58.33%;}
  .col-8 {width: 66.66%;}
  .col-9 {width: 75%;}
  .col-10 {width: 83.33%;}
  .col-11 {width: 91.66%;}
  .col-12 {width: 100%;}
}

ideally:

/* Extra small devices (phones, 600px and down) */
@media only screen and (max-width: 600px) {...}

/* Small devices (portrait tablets and large phones, 600px and up) */
@media only screen and (min-width: 600px) {...}

/* Medium devices (landscape tablets, 768px and up) */
@media only screen and (min-width: 768px) {...}

/* Large devices (laptops/desktops, 992px and up) */
@media only screen and (min-width: 992px) {...}

/* Extra large devices (large laptops and desktops, 1200px and up) */
@media only screen and (min-width: 1200px) {...}

to handle orientation:

@media only screen and (orientation: landscape) {
  body {
    background-color: lightblue;
  }
}

hide elements

/* If the screen size is 600px wide or less, hide the element */
@media only screen and (max-width: 600px) {
  div.example {
    display: none;
  }
}

variable font-size

/* If the screen size is 601px or more, set the font-size of <div> to 80px */
@media only screen and (min-width: 601px) {
  div.example {
    font-size: 80px;
  }
}

/* If the screen size is 600px or less, set the font-size of <div> to 30px */
@media only screen and (max-width: 600px) {
  div.example {
    font-size: 30px;
  }
}