Selector API in the CSS context.
When a user agent can't parse the selector (i.e., it is not valid CSS), it ignores the declaration block. The whole statement is ignored if there is an error anywhere in the selector.
It's an important element of the CSS addressing (selecting) model.
In CSS, the selector consists of everything up to (but not including) the first left curly brace ({) of a declaration block.
identifiers (including element names, classes, and IDs in selectors):
More, see the identifier specification
You need to use a full qualified selector expression
h1#2019-10-01 {
color:blue;
}
h1[id="2019-10-01"] {
color:blue;
}
<h1 id="2019-10-01">Hallo World</h1>