<!DOCTYPE html> minangka baris pisanan saka dokumen HTML. Mangkene browser kanggo nggayutake kanthi standards mode kaluaran quirks mode (mode selaras-bug lawas sing ngemulasi browser taun 1990an).
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Bebas doctype, browser nyebul maneh menyang quirks mode, sing ngganti tingkahe tata letak kanthi cara sing gumunantun — paling misale box model:
Quirks mode: width includes padding + border (old IE box model)
Standards mode: width is the content box (CSS spec) — predictable
Quirks liyane kalebu panggunaan beda saka line-height ing sel tabel, spasi gambar, lan unit CSS. Ora konsistenan iki nggawake CSS sing ditulis kanthi teliti nggayut beda-beda, dadi meh tansah pengin standards mode.
Ing HTML4/XHTML doctype minangka URL dawa sing ngrujuk DTD (Document Type Definition):
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "...dtd">
HTML5 dimadani dadi mung <!DOCTYPE html> — ora mangsuli menyang DTD maneh; iku mulane modar switch sing browser pesthine.
Lagi nglibatake <!DOCTYPE html> minangka baris pisanan.
Iku jaminan baris siji yen kaca sampeyan nganggo aturan nggayut modern, konsisten sing saben tutorial CSS lan framework nganggep — mung nglibatake nglibatake beda tata letak sing gampang bingung, angel di-debug.