Newline character sequence in CSS 'content' property?

The content property accepts a string and:

A string cannot directly contain a newline. To include a newline in a string, use an escape representing the line feed character in ISO-10646 (U+000A), such as "\A" or "\00000a". This character represents the generic notion of "newline" in CSS.

The tricky bit is to remember that HTML collapses white-space by default.

figure {
    /* Avoid whitespace collapse to illustrate what works and what doesn't */
    white-space: pre-wrap;
}

#first figcaption:before
{
    /* \n is not a valid entity in this context */
    content: 'Figure \n Chemistry';
    display: block;  
}

#second figcaption:before
{
    content: 'Figure \A Chemistry';
    display: block;  
}

<figure id='first'>
    <figcaption>Experiments</figcaption>
</figure>

<figure id='second'>
    <figcaption>Experiments</figcaption>
</figure>

You can check Using character escapes in markup and CSS for reference about the escape syntax, which essentially is:

• \20AC must be followed by a space if the next character is one of a-f, A-F, 0-9
• \0020AC must be 6 digits long, no space needed (but can be included)

NOTE: use \00000a rather than just \A when escaping an arbitrary string, because if the newline is followed by a number or any character from [a-f] range, this may give an undesired result.

figcaption:before
{
    content: 'Figure \a' attr(title);
    white-space: pre;
}

Note that in the content attribute value, concatenation is expressed just by whitespace, not by a “+” sign. The escape notation \a in a CSS string literal indicates a linebreak character.