Applying multiple elements
<!DOCTYPE html> will throw an error in the HTML5 validator. Any error on the web page has a direct or indirect effect on the search engine optimization of these webpages. In your case.
Parsing is a very significant process within the rendering engine.
Parsing a document means translating it to a structure the code can
use. The result of parsing is usually a tree of nodes that represent
the structure of the document. This is called a parse tree or a syntax
tree. Parsing is based on the syntax rules the document obeys: the
language or format it was written in. Every format you can parse must
have deterministic grammar consisting of vocabulary and syntax rules.
Parsing can be separated into two sub processes: lexical analysis and
Lexical analysis is the process of breaking the input into tokens.
Tokens are the language vocabulary: the collection of valid building
blocks. In human language it will consist of all the words that appear
in the dictionary for that language.
Syntax analysis is the applying of the language syntax rules.
Parsers usually divide the work between two components: the lexer
(sometimes called tokenizer) that is responsible for breaking the
input into valid tokens, and the parser that is responsible for
constructing the parse tree by analyzing the document structure
according to the language syntax rules.
The parsing process is iterative. The parser will usually ask the
lexer for a new token and try to match the token with one of the
syntax rules. If a rule is matched, a node corresponding to the token
will be added to the parse tree and the parser will ask for another
If no rule matches, the parser will store the token internally, and
keep asking for tokens until a rule matching all the internally stored
tokens is found. If no rule is found then the parser will raise an
exception. This means the document was not valid and contained syntax
Source: How Browsers Work: Behind the scenes of modern web browsers of HTML5 Rock.
You can see how much work the browser does when opening a webpage. This parsing takes some time for the browser. Now imagine that in the source code of open web pages have errors. Accordingly, the browser parsing time is increased. Accordingly, the speed of downloading the webpage decreases, but this is a signal of Google's search ranking - both for desktops and for mobile. Thus, breaking the standard of HTML (source code errors - this is a violation of the accepted standard) you are contributing a decrease in the search rank of your webpages, that is, a negative SEO.
A possible solution to this problem is the use of a tag code.