border-bottom of text format

I try to underline a paragraph with the following method:

```Xml
<DefineTextformat name="h" alignment="leftaligned" border-top="0.5pt" border-bottom="0.5pt" hyphenate="no" />
```

```Xml
<Output>
  <Text>
    <Paragraph textformat="h">
      <Value select="title" />
    </Paragraph>
    <Paragraph>
```
But I do not get any line.

Why is there no `margin-bottom`, `padding-bottom`?