CSS Live Demo: text-emphasis-position Click the demo button below!
/* 
    [ over | under ]:
    Specifies the vertical position of the emphasis marks.
    This part is required.

    [ right | left ]?:
    Specifies the horizontal position of the emphasis marks.
    This part is optional.
    If omitted, the initial value "right" is used.
*/
selector {
    text-emphasis-position: [ over | under ] && [ right | left ]?;
}
p span {
    text-emphasis-style: "z";
    text-emphasis-color: red;
    text-emphasis-position: under; /* The [ right | left ] keywords are optional and can be omitted */
    font-size: 1.5em;
}
<p>This text has an <span>emphasis mark</span>.</p>
Rendered Output
p span {
    writing-mode: vertical-rl; /* Vertical writing mode */
    text-emphasis-style: dot;
    text-emphasis-color: red;
    text-emphasis-position: over left;
    font-size: 1.5em;
}
<p lang="ja">これは<span>強調表示</span>されたテキストです。</p>
Rendered Output
p span {
    writing-mode: vertical-rl; /* Vertical writing mode */
    text-emphasis-style: dot;
    text-emphasis-color: red;
    text-emphasis-position: left; /* Will not apply */
    font-size: 1.5em;
}
Rendered Output The left value does not apply in vertical writing mode without an accompanying over or under value.