Twital allows you to create HTML/XML attributes in a very simple way.

t:attr-append is a different version of t:attr: it allows to append content to existing attributes instead of replacing it.

<div class="row" t:attr-append=" condition ? class=' even'">
     class will be "row even" if 'i' is odd.

In the same way of t:attr, condition and the value of attribute can be any valid Twig expression.

<div class="row"
    t:attr-append=" i mod 2 ? class=' even'|upper">
     class will be "row EVEN" if 'i' is odd.

When not needed, you can omit the condition instruction.

<div class="row" t:attr-append=" class=' even'">
     Class will be "row even"