Question

which is better for use

.menu{
    float:left;
    width:600px;
    height:25px;
    background:url(bg.png) repeat-x;

}
.menu ul{
    float:left;
}
.menu ul li{
    float:left;
    width:150px;
    height:25px;
    background:#F00;
}

or

.menu{
    float:left;
    width:600px;
    height:25px;
    background:url(bg.png) repeat-x;

}
.menu ul{
    float:left;
}
.menu li{
    float:left;
    width:150px;
    height:25px;
    background:#F00;
}

which tag is right menu ul li or menu li?

Answer 1

When you say which tag is right menu ul li or menu li?, are you talking about a div with class="menu" or are you talking about the deprecated menu tag (<menu>)?

If you are just talking about your css code, those are not tags, they are selectors. And I’d go with the most specific selector available in order to avoid accidental assignments

.menu > ul > li{
    // this matches only list items directly inside a ul directly inside a .menu
}

even better would be this:

#menu > ul > li{
    // now you are referencing the menu by id, so you know this is a unique assignment
}

or, if you have multiple menus:

#menubar > .menu > ul > li{
}

because otherwise you are in for surprises, you might actually have a structure like this:
(this is ugly, I know, but just to prove a point)

<div class="menu">
    <ul>
        <li>Menu Item 1</li>
        <li>Menu Item 2</li>
        <li>Menu Item 3
        <ul>
            <li id="abc">Menu Item abc</li>
        </ul>
        </li>
        <li>Menu Item 4
        <div><div><div><ol><li><div><ul>
                <li id="xyz">Menu Item xyz</li>
        </ul></div></li></ol></div></div></div>
        </li>
    </ul>
</div>

(you probably don’t want to match items abc or xyz).