CSS property value from class name

No it isn't. The closest we have to this is the attr() function, but that only works within the content property:

figure::before {
  content: attr(data-before) ', ';
}

figure::after {
  content: attr(data-after) '!';
}

<figure data-before="Hello" data-after="world"></figure>

Perhaps one day this will be expanded so that we can use it elsewhere, but for now this isn't possible.

Currently as I'm sure you're aware if you want to be able to use the .mrg-t-X class, you'll need to define separate style rules for each X you wish to allow:

.mrg-t-1 { ... }
.mrg-t-2 { ... }
.mrg-t-3 { ... }
...

Nowdays you can use CSS variable inside a style attribute instead generating a specific class:

Custom properties (sometimes referred to as CSS variables or cascading variables) are entities defined by CSS authors that contain specific values to be reused throughout a document. They are set using custom property notation (e.g., --main-color: black;) and are accessed using the var() function (e.g., color: var(--main-color);).

Complex websites have very large amounts of CSS, often with a lot of repeated values. For example, the same color might be used in hundreds of different places, requiring global search and replace if that color needs to change. Custom properties allow a value to be stored in one place, then referenced in multiple other places. An additional benefit is semantic identifiers. For example, --main-text-color is easier to understand than #00ff00, especially if this same color is also used in other contexts.

Custom properties are subject to the cascade and inherit their value from their parent.

example

  span {
  display: block;
  margin-top: var(--m-t);
}

html {
  background: repeating-linear-gradient(to bottom, transparent, 10px, lightgrey 10px, lightgrey 20px);} /* see 10px steps  */

<span style="--m-t:50px">one</span>
<span style="--m-t:85px">two</span>
<span style="--m-t:110px;">three</span>

Maybe you are looking for SCSS or LESS. It have mixins, variables, etc, and it autocompile real and long css. It was maded to this purposes and write less and less css with the same result.

http://sass-lang.com/guide

http://lesscss.org/

 @size: 10px;
 .class { font-size: @size;  }

Good luck!