Orientation of vertical text with unknown data at all

[Duquan]

Good day! I am interested in the possibility of centralizing a building block within an unknown height that is inside an unknown height. I mean, I've been reviewing a lot of resources and questions here, everyone's suggesting that if there's a parent's height or at least a parent's block, everything works. That's right. What if I have a CSS code like this:

#preloader {
    position:fixed;
    top:0;
    left:0;
    right:0;
    bottom:0;
    z-index: 9999;
}
.loader {
    position: absolute;
}

And this HTML:

<div id="preloader">
    <div class="loader">  
        <span id="loader_txt">Здесь текст</span>
    </div>
</div>

I didn't leave it on purpose in the code of implantation on the construction elements because it's obvious. I'm asking a little philosophical. If the heights are unknown, do you have to use JS? Thank you.

[rosemadder]

You can centralize content in "unknown" with help. https://developer.mozilla.org/ru/docs/Web/CSS/CSS_Flexible_Box_Layout ♪

In your case, it will be like this:

#preloader {
    position: fixed;
    top:0;
    left:0;
    right:0;
    bottom:0;
    z-index: 9999;
    background: rgba(0, 0, 0, .7);
display: flex; /* Магия */
align-items: center; /* Центрируем по вертикали */
justify-content: center; /* Центрируем по горизонтали */

}

Demo: http://jsfiddle.net/42f10edL/1/

Several interesting resources for flexbox:

1. http://habrahabr.ru/post/242545/
2. http://caniuse.com/#feat=flexbox
3. https://css-tricks.com/snippets/css/a-guide-to-flexbox/