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<form ... > tag is embedded in an jsp @include file... </form> follows the @include in the .jsp file. The code compiles correctly and is HTML strict compliant when executed. If the @include file is inserted into the .jsp file, the warning disapears. It appears that the HTML line editor is not looking at the HTML code in the @include file.
You are right, the matching checker operates just on the opened file. This issue is valid and should be addressed. Some high level dependencies model is planned for 7.0 so this may be addressed as a part of this.