How to load a stylesheet into wp_head from a custom widget?

There are various approaches (including this one), but you could consider, since you’re talking about a stylesheet and not a script, simply enqueueing it from the Plugin, regardless of whether the Widget is active. (Which is worse: the few bytes of Widget-specific CSS, or the server overhead to determine if the Widget is active?)

Just add the following to your Plugin file (outside of the Widget class definition):

<?php
function plugin-name_print_stylesheet() {
    ?>
    <style type="text/css">
    /* CSS definitions go here */
    </style>
    <?php
}
add_action( 'wp_print_styles', 'plugin-name_print_stylesheet' );
?>

Or, if you’ve got a .css file with your Widget’s stylesheet:

<?php
function plugin-name_enqueue_stylesheet() {
    wp_enqueue_style( 'plugin-name-stylesheet', plugins_dir( 'stylesheet.css', __FILE__ ) );
}
add_action( 'wp_enqueue_scripts', 'plugin-name_enqueue_stylesheet' );
?>

p.s. don’t use a generic term such as my_slider as a custom-function prefix. You should use your plugin-slug instead, as a prefix for all of your Plugin’s custom functions.

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