Css – What does ‘&.’ in ‘&.sub-title’ indicates in scss

You can check directly at the CSS grammar.

Basically¹, a name must begin with an underscore (_), a hyphen (-), or a letter(a–z), followed by any number of hyphens, underscores, letters, or numbers. There is a catch: if the first character is a hyphen, the second character must² be a letter or underscore, and the name must be at least 2 characters long.

-?[_a-zA-Z]+[_a-zA-Z0-9-]*

In short, the previous rule translates to the following, extracted from the W3C spec.:

In CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-z0-9] and ISO 10646 characters U+00A0 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, or a hyphen followed by a digit. Identifiers can also contain escaped characters and any ISO 10646 character as a numeric code (see next item). For instance, the identifier "B&W?" may be written as "B&W?" or "B\26 W\3F".

Identifiers beginning with a hyphen or underscore are typically reserved for browser-specific extensions, as in -moz-opacity.

¹ It's all made a bit more complicated by the inclusion of escaped unicode characters (that no one really uses).

² Note that, according to the grammar I linked, a rule starting with TWO hyphens, e.g. --indent1, is invalid. However, I'm pretty sure I've seen this in practice.

Relative+absolute positioning is your best bet:

#header {
  position: relative;
  min-height: 150px;
}

#header-content {
  position: absolute;
  bottom: 0;
  left: 0;
}

#header, #header * {
  background: rgba(40, 40, 100, 0.25);
}

<div id="header">
  <h1>Title</h1>
  <div id="header-content">And in the last place, where this might not be the case, they would be of long standing, would have taken deep root, and would not easily be extirpated. The scheme of revising the constitution, in order to correct recent breaches of it, as well as for other purposes, has been actually tried in one of the States.</div>
</div>

But you may run into issues with that. When I tried it I had problems with dropdown menus appearing below the content. It's just not pretty.

Honestly, for vertical centering issues and, well, any vertical alignment issues with the items aren't fixed height, it's easier just to use tables.

Example: Can you do this HTML layout without using tables?