You can use DATA tag like this code below:

File: code/[Package]/[Module]/etc/adminhtml/system.xml

<field id="pattern" translate="label comment" type="text" sortOrder="3" showInDefault="1" showInWebsite="1" showInStore="1">
    <label>Gift code pattern</label>       
    <comment><![CDATA[Generate gift codes for Gift Card products. For example:<br/><strong>[A.8] : 8 alpha<br/>[N.4] : 4 numeric<br/>[AN.6] : 6 alphanumeric<br/>GIFT-[A.4]-[AN.6] : GIFT-ADFA-12NF0O</strong>]]></comment>
</field>

You can use CDATA tag like this code below:

File: code/[Package]/[Module]/etc/adminhtml/system.xml

<field id="pattern" translate="label comment" type="text" sortOrder="3" showInDefault="1" showInWebsite="1" showInStore="1">
    <label>Gift code pattern</label>       
    <comment><![CDATA[Generate gift codes for Gift Card products. For example:<br/><strong>[A.8] : 8 alpha<br/>[N.4] : 4 numeric<br/>[AN.6] : 6 alphanumeric<br/>GIFT-[A.4]-[AN.6] : GIFT-ADFA-12NF0O</strong>]]></comment>
</field>