How to change text transparency in HTML/CSS?

Just use the rgba tag as your text color. You could use opacity, but that would affect the whole element, not just the text. Say you have a border, it would make that transparent as well.

.text
    {
        font-family: Garamond, serif;
        font-size: 12px;
        color: rgba(0, 0, 0, 0.5);
    }

opacity applies to the whole element, so if you have a background, border or other effects on that element, those will also become transparent. If you only want the text to be transparent, use rgba.

#foo {
    color: #000; /* Fallback for older browsers */
    color: rgba(0, 0, 0, 0.5);

    font-size: 16pt;
    font-family: Arial, sans-serif;
}

Also, steer far, far away from <font>. We have CSS for that now.

Check Opacity, You can set this css property to the div, the <p> or using <span> you want to make transparent

And by the way, the font tag is deprecated, use css to style the text

div {
    opacity: 0.5;
} 

Edit: This code will make the whole element transparent, if you want to make ** just the text ** transparent check @Mattias Buelens answer