Disallow certain blocking DOM calls during microtask execution.

third_party/WebKit/Source/bindings/IDLExtendedAttributes.md

 # Blink IDL Extended Attributes
 
 [TOC]
 
 ## Introduction
 
 The main interest in extended attributes are their _semantics_: Blink implements many more extended attributes than the Web IDL standard, to specify various behavior.
 
 The authoritative list of allowed extended attributes and values is [bindings/IDLExtendedAttributes.txt](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/bindings/IDLExtendedAttributes.txt). This is complete but not necessarily precise (there may be unused extended attributes or values), since validation is run on build, but coverage isn't checked.
 
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 This is similar to the standard `[DeprecateAs]` extended attribute, but does not display a deprecation warning.
 
 Usage: `[MeasureAs]` can be specified on methods, attributes, and constants. The value must match one of the enumeration values in [UseCounter::Feature](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/core/frame/UseCounter.h&q=%22enum%20Feature%22&sq=package:chromium&type=cs&l=61) (in [core/frame/UseCounter.h](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/core/frame/UseCounter.h)).
 
 ```webidl
 [MeasureAs=AttributeWeAreInterestedIn] attribute Node interestingAttribute;
 [MeasureAs=MethodsAreInterestingToo] Node getInterestingNode();
 [MeasureAs=EvenSomeConstantsAreInteresting] const INTERESTING_CONSTANT = 1;
 ```
 
+### [NotDuringMicrotasks] _(m)_
+
+Summary: A method marked as `[NotDuringMicrotasks]` will do nothing and return
+undefined if invoked during microtask execution.
+
+Usage: `[NotDuringMicrotasks]` can be specified on methods.
+
+```webidl
+[NotDuringMicrotasks] void alert();
+```
+
 ### [NotEnumerable] _(m, a, s)_
 
 *** note
 **FIXME:** docs out of date!
 ***
 
 Specification: [The spec of Writable, Enumerable and Configurable (Section 8.6.1)](http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf) - _not standard Web IDL extended attributes._
 
 Summary: Controls the enumerability of methods and attributes.
 
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 Copyright (C) 2009 Apple Inc. All rights reserved.
 
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 ***