Giving a border to an HTML table row, <tr>

Absolutely! Just use

<tr style="outline: thin solid">

on which ever row you like. Here's a fiddle.

Of course, as people have mentioned, you can do this via an id, or class, or some other means if you wish.

You can use the box-shadow property on a tr element as a subtitute for a border. As a plus, any border-radius property on the same element will also apply to the box shadow.

box-shadow: 0px 0px 0px 1px rgb(0, 0, 0);

You can set border properties on a tr element, but according to the CSS 2.1 specification, such properties have no effect in the separated borders model, which tends to be the default in browsers. Ref.: 17.6.1 The separated borders model. (The initial value of border-collapse is separate according to CSS 2.1, and some browsers also set it as default value for table. The net effect anyway is that you get separated border on almost all browsers unless you explicitly specifi collapse.)

Thus, you need to use collapsing borders. Example:

<style>
table { border-collapse: collapse; }
tr:nth-child(3) { border: solid thin; }
</style>

Yes. I updated my answer DEMO

table td {
    border-top: thin solid; 
    border-bottom: thin solid;
}

table td:first-child {
     border-left: thin solid;
}

table td:last-child {
     border-right: thin solid;
}

If you want to style only one <tr> you can do it with a class: Second DEMO