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.size-auto,
.size-full,
.size-large,
.size-medium,
.size-thumbnail {
max-width: 100%;
height: auto;
}

/* That will change the video width to 600px instead of the WordPress default. */

<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.

if ( ! isset( $content_width ) ) $content_width = 600;