固定位置和浮子无法正常工作

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    <h1><span class="mw-headline" id="The_third_dimension.E2.80.94z-index">The third dimension—z-index</span></h1>
    <p>It’s natural to regard a web page as two-dimensional. Technology hasn’t evolved far enough that 3D displays are commonplace, so we have to be content with width and height and fake 3D effects. But CSS rendering actually happens in three dimensions!
      That doesn’t mean you can make an element hover in front of the monitor—yet—but you can do some other useful things with positioned elements.
    </p>
    <p>The two main axes in a web page are the horizontal X axis and the vertical Y axis. The origin of this co-ordinate system is in the upper left-hand corner of the viewport, ie where both the X and Y values are 0. But there is also a Z axis, which we
      can imagine as running perpendicular to the monitor’s surface (or to the paper, when printing). Higher Z values indicate a position “in front of” lower Z values. Z values can also be negative, which indicate a position “behind” some point of reference
      (I’ll explain this point of reference in a minute).
    </p>
    <p>Before we continue, I should warn you that this is one of the most complicated topics within CSS, so don’t get disheartened if you don't understand it on your first read.
    </p>
    <p>Positioned elements (including relatively positioned elements) are rendered within something known as a stacking context. Elements within a stacking context have the same point of reference along the Z axis. I’ll explain more about this below. You
      can change the Z position (also called the stack level) of a positioned element using the <code>z-index</code> property. The value can be an integer number (which may be negative) or one of the keywords <code>auto</code> or <code>inherit</code>. The
      default value is <code>auto</code>, which means the element has the same stack level as its parent.
    </p>
    <p>You should note that you can only specify an <i>index</i> position along the Z axis. You can’t make one element appear 19 pixels behind or 5 centimetres in front of another. Think of it like a deck of cards: you can stack the cards and decide that the
      ace of spades should be on top of the three of diamonds—each card has its stack level, or Z index.
    </p>
    <p>If you specify the <code>z-index</code> as a positive integer, you assign it a stack level “in front of” other elements within the same stacking context that have a lower stack level. A <code>z-index</code> of 0 (zero) means the same as <code>auto</code>,
      but there is a difference to which I will come back in a minute. A negative integer value for <code>z-index</code> assigns a stack level “behind” the parent’s stack level.
    </p>
    <p>When two elements in the same stacking context have the same stack level, the one that occurs later in the source code will appear on top of its preceding siblings.
    </p>
    <p>There can in fact be no less than seven layers in one stacking context, and any number of elements in those layers, but don't worry—you are unlikely to have to deal with seven layers in a stacking context. The order in which the elements (all elements,
      not only the positioned ones) within one stacking context are rendered, from back to front is as follows:
    </p>
    <ol>
      <li>The background and borders of the elements that form the stacking context
      </li>
      <li> <b>Positioned descendants with negative stack levels</b>
      </li>
      <li>Block-level descendants in the normal flow
      </li>
      <li>Floated descendants
      </li>
      <li>Inline-level descendants in the normal flow
      </li>
      <li> <b>Positioned descendants with the stack level set as <code>auto</code> or  (zero)</b>
      </li>
      <li> <b>Positioned descendants with positive stack levels</b>
      </li>
    </ol>
    <p>The highlighted entries are the elements whose stack level we can change using the <code>z-index</code> property.
    </p>
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    <h3>This is a fixed menu.</h3>
    <ul>
      <li><a href="#">Link 1</a>
      </li>
      <li><a href="#">Link 2</a>
      </li>
      <li><a href="#">Link 3</a>
      </li>
      <li><a href="#">Link 4</a>
      </li>
      <li><a href="#">Link 5</a>
      </li>
    </ul>

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