Transparency in CSS

Problem

On a web page an HTML element is placed on another and the first one wish to look and feel transparent.

Solution

The CSS has a declaration called opacity.

Example

We have an example HTML page with solid background color and a button placed on that.

HTML

<html DOCTYPE!>
 <head>
     <style>
         #main {
             background-color: yellow;
             height: 100%;
         }
     </style>
 </head>
 <body>
     <div id="main">
         <input type="button" id="btnTransparent" value="Test"/>
     </div>
 </body>
 </html>

The code looks like this:

Now, let’s make the button transparent. We will select it by it’s id: btnTransparent and add some transparency.

#btnTransparent {
    opacity: .5;
}

Put the code together:

<html DOCTYPE!>
 <head>
     <style>
         #main {
             background-color: yellow;
             height: 100%;
         }
         #btnTransparent {
             opacity: .5;
         }
     </style>
 </head>
 <body>
     <div id="main">
         <input type="button" id="btnTransparent" value="Test"/>
     </div>
 </body>
 </html>

It looks like this:

The Test button appears half transparent as set by opacity: .5; It accepts a value from 0 to 1 meaning 0 is full transparent, not visible.

Let’s now add a pseudo element on the button, when the mouse hovers the button, the opacity returns to full.

This would do the trick

#btnTransparent:hover {
 opacity: 1;
}

And the complete code:

<html DOCTYPE!>
 <head>
     <style>
         #main {
             background-color: yellow;
             height: 100%;
         }
         #btnTransparent {
             opacity: .5;
         }
          #btnTransparent:hover{
             opacity: 1;  
         }
      </style>
 </head>

 <body>
     <div id="main">
         <input type="button" id="btnTransparent" value="Test"/>
     </div>
 </body>
 </html>

Result

Now, let’s put it all together with a nice background image as follows:

<html DOCTYPE!>
 <head>
     <style>
         * {
            margin: 0;
            padding: 0;
         }
         #title_area{
             position: absolute;
             top: 200px;
             left: 50px;
 
             width: 350px;
             height: 100px;
 
             display: flex;
             justify-content: center;
             align-items: center; 
             flex-direction: column; 
 
             background-color: sandybrown;
             opacity: .5;
         }
      </style>
 </head>
 <body>
    <div id="title_area"><h3>
        <h3>Transparent or not transparent<h3/>
    </div>
    <img src="background.jpg" width="500" height="600"/>
 </body>
 </html>
Result

Background image taken from here

Conclusion

Pretty neat. A simple CSS declaration changes the look and feel of the UI.

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