b elementHTMLElement.The b element represents a span of text
  to which attention is being drawn for utilitarian purposes without
  conveying any extra importance and with no implication of an
  alternate voice or mood, such as key words in a document abstract,
  product names in a review, actionable words in interactive
  text-driven software, or an article lede.
The following example shows a use of the b element
   to highlight key words without marking them up as important:
<p>The <b>frobonitor</b> and <b>barbinator</b> components are fried.</p>
In the following example, objects in a text adventure are
   highlighted as being special by use of the b
   element.
<p>You enter a small room. Your <b>sword</b> glows brighter. A <b>rat</b> scurries past the corner wall.</p>
Another case where the b element is appropriate is
   in marking up the lede (or lead) sentence or paragraph. The
   following example shows how a BBC
   article about kittens adopting a rabbit as their own could be
   marked up:
<article> <h2>Kittens 'adopted' by pet rabbit</h2> <p><b class="lede">Six abandoned kittens have found an unexpected new mother figure — a pet rabbit.</b></p> <p>Veterinary nurse Melanie Humble took the three-week-old kittens to her Aberdeen home.</p> [...]
As with the i element, authors can use the class attribute on the b
  element to identify why the element is being used, so that if the
  style of a particular use is to be changed at a later date, the
  author doesn't have to go through annotating each use.
The b element should be used as a last resort when
  no other element is more appropriate. In particular, headings should
  use the h1 to h6 elements, stress emphasis
  should use the em element, importance should be denoted
  with the strong element, and text marked or highlighted
  should use the mark element.
The following would be incorrect usage:
<p><b>WARNING!</b> Do not frob the barbinator!</p>
In the previous example, the correct element to use would have
   been strong, not b.
Style sheets can be used to format b
  elements, just like any other element can be restyled. Thus, it is
  not the case that content in b elements will
  necessarily be boldened.