doc.css

/* ####################
   ## SELECTORS
   #################### */

/* the selector is used to target an element on a page. */
selector { property: value; /* more properties...*/ }

/*
Here is an example element:

<div class='class1 class2' id='anID' attr='value' otherAttr='en-us foo bar' />
*/

/* You can target it using one of its CSS classes */
.class1 { }

/* or both classes! */
.class1.class2 { }

/* or its name */
div { }

/* or its id */
#anID { }

/* or using the fact that it has an attribute! */
[attr] { font-size:smaller; }

/* or that the attribute has a specific value */
[attr='value'] { font-size:smaller; }

/* starts with a value (CSS 3) */
[attr^='val'] { font-size:smaller; }

/* or ends with a value (CSS 3) */
[attr$='ue'] { font-size:smaller; }

/* or contains a value in a space-separated list */
[otherAttr~='foo'] { }
[otherAttr~='bar'] { }

/* or contains a value in a dash-separated list, e.g., "-" (U+002D) */
[otherAttr|='en'] { font-size:smaller; }


/* You can combine different selectors to create a more focused selector. Don't
   put spaces between them. */
div.some-class[attr$='ue'] { }

/* You can select an element which is a child of another element */
div.some-parent > .class-name { }

/* or a descendant of another element. Children are the direct descendants of
   their parent element, only one level down the tree. Descendants can be any
   level down the tree. */
div.some-parent .class-name { }

/* Warning: the same selector without a space has another meaning.
   Can you guess what? */
div.some-parent.class-name { }

/* You may also select an element based on its adjacent sibling */
.i-am-just-before + .this-element { }

/* or any sibling preceding it */
.i-am-any-element-before ~ .this-element { }

/* There are some selectors called pseudo classes that can be used to select an
   element only when it is in a particular state */

/* for example, when a link hasn't been visited */
selected:link { }

/* or a link has been visited */
selector:visited { }

/* or an element is in focus */
selected:focus { }

/* or when the cursor hovers over an element */
selector:hover { }

/* or when a link is clicked on */
selector:active { }

/* These pseudo classes regarding links should always be written in the above order or the code might not work as expected */

/* Any element that is the first child of its parent */
selector:first-child {}

/* any element that is the last child of its parent */
selector:last-child {}

/* Just like pseudo classes, pseudo elements allow you to style certain parts of
    a document  */

/* matches a virtual first child of the selected element */
selector::before {}

/* matches a virtual last child of the selected element */
selector::after {}

/* At appropriate places, an asterisk may be used as a wildcard to select every
   element */
* { } /* all elements */
.parent * { } /* all descendants */
.parent > * { } /* all children */

/* Group any number of selectors to define styles that affect all selectors
   in the group */
selector1, selector2 { }

/* ####################
   ## PROPERTIES
   #################### */

selector {

    /* Units of length can be absolute or relative. */

    /* Relative units */
    width: 50%;       /* percentage of parent element width */
    font-size: 2em;   /* multiples of element's original font-size */
    font-size: 2rem;  /* or the root element's font-size */
    font-size: 2vw;   /* multiples of 1% of the viewport's width (CSS 3) */
    font-size: 2vh;   /* or its height */
    font-size: 2vmin; /* whichever of a vh or a vw is smaller */
    font-size: 2vmax; /* or greater */

    /* Absolute units */
    width: 200px;     /* pixels */
    font-size: 20pt;  /* points */
    width: 5cm;       /* centimeters */
    min-width: 50mm;  /* millimeters */
    max-width: 5in;   /* inches */

    /* Colors */
    color: #F6E;                    /* short hex format */
    color: #FF66EE;                 /* long hex format */
    color: tomato;                  /* a named color */
    color: rgb(255, 255, 255);      /* as rgb values */
    color: rgb(10%, 20%, 50%);      /* as rgb percentages */
    color: rgba(255, 0, 0, 0.3);    /* as rgba values (CSS 3) Note: 0 <= a <= 1 */
    color: transparent;             /* equivalent to setting the alpha to 0 */
    color: hsl(0, 100%, 50%);       /* as hsl percentages (CSS 3) */
    color: hsla(0, 100%, 50%, 0.3); /* as hsl percentages with alpha */

    /* Borders */
    border-width:5px;
    border-style:solid;
    border-color:red;      /* similar to how background-color is set */
    border: 5px solid red; /* this is a short hand approach for the same */
    border-radius:20px;    /* this is a CSS3 property */

    /* Images as backgrounds of elements */
    background-image: url(/img-path/img.jpg); /* quotes inside url() optional */

    /* Fonts */
    font-family: Arial;
    /* if the font family name has a space, it must be quoted */
    font-family: "Courier New";
    /* if the first one is not found, the browser uses the next, and so on */
    font-family: "Courier New", Trebuchet, Arial, sans-serif;
}

 /* You need to include the css file in your page's <head>. This is the
     recommended method.  */
<link rel='stylesheet' type='text/css' href='path/to/style.css'>

/* You can also include some CSS inline in your markup. */
<style>
   a { color: purple; }
</style>

/* Or directly set CSS properties on the element. */
<div style="border: 1px solid red;">
</div>

/* A */
p.class1[attr='value']

/* B */
p.class1 { }

/* C */
p.class2 { }

/* D */
p { }

/* E */
p { property: value !important; }

<p style='/*F*/ property:value;' class='class1 class2' attr='value'>

/* A rule that will be used on all devices */
h1 {
  font-size: 2em;
  color: white;
  background-color: black;
}

/* change the h1 to use less ink on a printer */
@media print {
  h1 {
    color: black;
    background-color: white;
  }
}

/* make the font bigger when shown on a screen at least 480px wide */
@media screen and (min-width: 480px) {
  h1 {
    font-size: 3em;
    font-weight: normal;
  }
}

<head>
  <meta name="viewport" content="width=device-width; initial-scale=1.0">
</head>