Why is wp_kses not keeping style attributes as expected?

This is an older question, but here’s the answer for future generations:

WordPress will check the styles against a list of allowed properties and it will still strip the style attribute if none of the styles are safe. The default allow list is:

• text-align
• margin
• color
• float
• border
• background
• background-color
• border-bottom
• border-bottom-color
• border-bottom-style
• border-bottom-width
• border-collapse
• border-color
• border-left
• border-left-color
• border-left-style
• border-left-width
• border-right
• border-right-color
• border-right-style
• border-right-width
• border-spacing
• border-style
• border-top
• border-top-color
• border-top-style
• border-top-width
• border-width
• caption-side
• clear
• cursor
• direction
• font
• font-family
• font-size
• font-style
• font-variant
• font-weight
• height
• letter-spacing
• line-height
• margin-bottom
• margin-left
• margin-right
• margin-top
• overflow
• padding
• padding-bottom
• padding-left
• padding-right
• padding-top
• text-decoration
• text-indent
• vertical-align
• width

This list is, as with most things in WordPress, filtered! You can add display to it as follows to let your code work as expected:

add_filter( 'safe_style_css', function( $styles ) {
    $styles[] = 'display';
    return $styles;
} );