Set CSS Border Color to text color

You actually get this behavior for free; it’s mentioned in the spec:

If an element’s border color is not specified with a border property, user agents must use the value of the element’s ‘color’ property as the computed value for the border color.

So all you have to do is omit the color when using the border shorthand property:

.dotted-underline {
    border-bottom: dotted 1px;
}

Or only use the border-style and border-width properties, and not border-color:

.dotted-underline {
    border-bottom-style: dotted;
    border-bottom-width: 1px;
}

Or, in browsers that support the new CSS3 keyword currentColor, specify that as the value for border-color (useful for overriding existing border-color declarations):

.dotted-underline {
    border-bottom-color: currentColor;
    border-bottom-style: dotted;
    border-bottom-width: 1px;
}

The color that the border takes on will be the same as the text color by default.