How to work out EM based media query

问题:

I'm trying to work out how to use EM media queries in my latest project. However after some testing I've found that the media queries are ever so slightly off and I can't work out why. It might have something to do with it using the parents font size instead of the body. My body is set to 14px and my workings out look like:

$break-small: 22.8571em; //320px
$break-smallish: 40em; //560px
$break-med: 54.8571em; //768px
$break-medish: 68.5714em; //960px
$break-desk: 73.1428em; //1024px

body font size:

body{
    font-size: 14px;
    line-height: 1.5;
    min-height: 100%;
}

*(from my SCSS breakpoint variables) From what I understand I did: 768 / 14 (base font size) = width in em's

Say I've a div called header, there is no font-size set on this div, only children of this div. Surely it would still then use the body font-size?

回答1:

Ems in media queries are never based on the font size of body, or any other element for that matter. They always refer to the default font size set by the user in the browser preferences. In most browsers this default font size is around 16px, and in CSS this corresponds to the initial value of the font-size property which is medium. From the spec:

Relative units in media queries are based on the initial value, which means that units are never based on results of declarations. For example, in HTML, the ‘em’ unit is relative to the initial value of ‘font-size’.

This same default font size is inherited by the root element, which is html, not body (see here). Specifying a relative font size on body just means body bases its own calculation on the computed font size of html. This being stated, note that setting font-size on html will not affect how ems are calculated in media queries either.

回答2:

Your guesswork is correct, the em unit sets the font size relative to the parent element's font-size, not relative to the document root. If you're looking for the latter you're looking for the rem unit, but browser support might be a problem for you, depending on your application.

See the following Fiddle for a sample: http://jsfiddle.net/afp46/

HTML:

<span >This is text</span>

<div><span >This is text</span></div>

<span><span>This is text</span></span>

CSS:

body {
    font-size: 14px;
}

div {
    font-size: 16px;
}

span {
    font-size: 1.2em;
}

回答3:

I would change that to body font size 100% and then you have the flexibility of EMs and %s site wide

回答4:

I would highly recommend you do font-sizing with rem, which stands for "root em". It's much more consistent. Read more about it here: http://snook.ca/archives/html_and_css/font-size-with-rem

Also, I would recommend adding this to your CSS:

html {
    font-size: 62.5%;
}

Now, your rems or ems will be easy to convert. 10px font-size would be 1rem or 1em. Nice, right? :) Again, use rems, its a much better practice these days.

回答5:

Please try this code. I have already used this my last project it working in fine. so please try.

// Small screens
@media only screen { } /* Define mobile styles */

@media only screen and (max-width: 40em) { } /* max-width 640px, mobile-only styles, use when QAing mobile issues */

// Medium screens
@media only screen and (min-width: 40.063em) { } /* min-width 641px, medium screens */

@media only screen and (min-width: 40.063em) and (max-width: 64em) { } /* min-width 641px and max-width 1024px. */

// Large screens
@media only screen and (min-width: 64.063em) { } /* min-width 1025px, large screens */

@media only screen and (min-width: 64.063em) and (max-width: 90em) { } /* min-width 1025px and max-width 1440px  */

@media only screen and (min-width: 90.063em) { } /* min-width 1441px, xlarge screens */

@media only screen and (min-width: 90.063em) and (max-width: 120em) { } /* min-width 1441px and max-width 1920px */

// XXLarge screens
@media only screen and (min-width: 120.063em) { } /* min-width 1921px, xxlarge screens */