[Bug] Cannot find element only be attribute name

[oleg-andreyev]

It should be possible to find tags only by attribute name: [attr-name]
Right now it's impossible because:

if ($rule['key'] != 'plaintext' && is_null($child->getAttribute($rule['key']))) {
  $pass = false;
}

Is forcing that have an value

[maxstar]

Actual problem: you can't find tag by attribute if this attribute does not have value.
<any-tag attribute>something</any-tag>

[paquettg]

merged PR