Each pixel represents one dot in the browser, which is why they’re also known as “dot-density units”. Pixel values are set with CSS and appear before or after any other unit like em or rem in your stylesheet.

What is exactly CSS value?

CSS value is a number that is used to set the size of text, images, and other elements on a web page. CSS values can be any unit that the browser supports. The most common units are pixels (px), centimeters (cm), millimeters (mm), inches (in), points (pt), picas (pc) and exes. For example:

p { font-size: 16px; }

font-size: 125% would make the font larger than if you had just used p { font-size: 12px; }

You can also use percentages with your CSS values in order to make things scale proportionally by changing one value rather than having to change all of them individually. For example: h1 { margin-top: -50%; } will move up half way so there's no space between paragraphs on your page anymore!

The CSS unit of measurement is px (pixels). These are the smallest units available in CSS and are used to set things like font size, image dimensions and widths.

The root element's font-size

The root element's font-size is 1rem. This value is inherited by all child elements, unless otherwise specified. The html element's font-size is 1rem and it's inherited by the body element. The p element's font-size is 0.8125rem (80%), which means that each pixel will be converted into 8.125 pixels, so if you set a pixel size for the paragraph, it will be displayed as 80% of its initial size on both desktop and mobile devices.

The div element has no default value set for its font-size property; however, if you set any other CSS properties on this type of HTML tag (e.g., background color), they will be applied differently depending on whether your webpage has been rendered through desktop or mobile browsers—this can lead to unexpected results!

REM and EM Differences in CSS

  • REM is relative to the font-size of the root element (the html tag)
  • EM is relative to the font-size of the parent element.

The reason for this difference is that HTML has no way to control how text will be rendered on different devices, so it leaves it up to CSS. 

The problem with EM is that if you set your body font-size as 12pt, then all your EMs will also be 12pt because there is no way for them to inherit from something else. 

In contrast, REM can be inherited and thus overridden by a higher level selector; in other words, if you have a paragraph which has been set with 1em margins and padding in its CSS then any elements inside it will inherit those margins and paddings unless there are separate rules defined explicitly for those elements at a higher level in the cascade.

Modern CSS measurements

Rem and em are the modern CSS measurements. Rem is relative to the root element's font-size, whereas em is relative to the current element's font-size.

Pixels are not relative to anything, which means that if you were to change the size of your browser window, pixels would not change at all. This makes px useful for both mobile and desktop websites (although it’s better used on responsive websites). Pixels are based on the CSS pixel unit, which can be defined as 1/96th of an inch or 96 per inch (1px = 1/96in). 

There are two different kinds of pixels: real pixels and reference pixels. That’s why we need a px converter!

So the answer to your question is that REM is more powerful than EM because it can be inherited.

Mobile Responsive Design Tricks

Like most things in life, there's a best way to do responsive web design. The best way is the rem unit.

Rem stands for root em, and it's an absolute measurement—it will maintain its size relative to the element or root element where it's set. 

It has no bearing on the rest of your website; it just stays the same size wherever you put it. 

If you want something to be 25px tall on mobile devices but change from 40px to 60px on larger screens, rem makes that easy because it will always stay at 25px no matter how big or small your screen gets.

Now let's consider ems as another option: They're relative units that are based off of whatever font-size has been set by default on any given HTML element (like body). 

So if I gave body a font size of 16px and then used 1em elsewhere in my code, everything I wrote would be 16px until further adjustments were made throughout my site (e.g., changing body from 16px to 24px would make all other elements 24px). 

Just like rems don't care about their surroundings when making measurements for themselves (unlike px), ems don't care about anything else when making measurements for themselves (unlike px).

The px converter will help you with both. You can convert between em, rem and px with ease. It’s also possible to convert between different units of length (px, cm, mm etc.)

This is why ems are so powerful and useful: They're always relative to something, which means they can be used anywhere on your website and will always maintain their size..

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