I have created a simple XML and a DTD, but for some reason every single validator I try keep giving me "No declaration for element" errors on every single line. I am not quite sure what I have done wrong as I have only just started looking into XML and DTD, and would appreciate some insight, so I know what I have done wrong and can improve. Any and all help would be greatly appreciated so I know what to do in the future.
<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT BILLER (RESTAURANTNAME,ADRESS,COMPANYPHONE,CVR)>
<!ELEMENT TIME (DATE,CLOCK)>
<!ELEMENT ORDER (MENUITEM+,CUSTOMER)>
<!ELEMENT RESTAURANTNAME (#PCDATA)>
<!ELEMENT ADRESS (#PCDATA)>
<!ELEMENT COMPANYPHONE (#PCDATA)>
<!ELEMENT CVR (#PCDATA)>
<!ELEMENT DATE (#PCDATA)>
<!ELEMENT CLOCK (#PCDATA)>
<!ELEMENT MENUITEM (FOOD,REQUEST,REQUESTPRICE?,QUANTITY,PRICETOTAL)>
<!ELEMENT CUSTOMER (TABLE,NAME,SPECIALTYPING,PAYMENTMETHOD)>
<!ELEMENT FOOD (#PCDATA)>
<!ELEMENT REQUEST (#PCDATA)>
<!ELEMENT REQUESTPRICE (#PCDATA)>
<!ELEMENT QUANTITY (#PCDATA)>
<!ELEMENT PRICETOTAL (#PCDATA)>
<!ELEMENT TABLE (#PCDATA)>
<!ELEMENT NAME (#PCDATA)>
<!ELEMENT SPECIALTYPING (#PCDATA)>
<!ELEMENT PAYMENTMETHOD (#PCDATA)>
]>
<regning id="122">
<tidspunkt>
<dato>03</dato>
<måned>01</måned>
<år>2020</år>
<tid>13.40</tid>
</tidspunkt>
<restaurant>
<restaurantnavn>Restaurant Fixi</restaurantnavn>
<adresse>Havstreet 128, 478 Andeby</adresse>
<telefon>81 19 26 18</telefon>
<cvr>29 23 28 00</cvr>
</restaurant>
<kundeinfo>
<bord>3</bord>
<kunde-nr>122</kunde-nr>
<specielt>vegetar</specielt>
<betaling>KONTANT</betaling>
</kundeinfo>
<tjener>
<fornavn>Lotti</fornavn>
<efternavn>Andersen</efternavn>
</tjener>
<ordre>
<menugenstand>
<madtitel>Vegetar Lakseburger med fritter</madtitel>
<ekstra>x</ekstra>
<quantity>1</quantity>
<pris valuta="DKK">80</pris>
</menugenstand>
<menugenstand>
<drikketitel>Økologisk hvedeøl</drikketitel>
<ekstra>x</ekstra>
<quantity>1</quantity>
<pris valuta="DKK">45</pris>
</menugenstand>
<menugenstand>
<madtitel>Båd Banannasplit</madtitel>
<ekstra>Ekstra flødeskum</ekstra>
<requestprice>10</requestprice>
<quantity>1</quantity>
<pris valuta="DKK">55</pris>
</menugenstand>
</ordre>
</regning>