The b element represents a span of text to be stylistically offset from the normal prose without conveying any extra importance, such as key words in a document abstract, product names in a review, or other spans of text whose typical typographic presentation is boldened. The following example shows a use of the b element to highlight key words without marking them up as important

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 using HTML5 elements:

<article>
    <h2>Kittens 'adopted' by pet rabbit</h2>
    <p><b>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>
    [...]

The b element in HTML 5

File under head-fuck. Depending on the alignment on the planets, this redefinition of b either makes some kind of perfect pragmatic sense to me, or causes my head to explode in a fit of presentation/content separation violation.

The idea of marking terms as ‘highlighted’ seems pretty reasonable. The second example seems entirely stylistic and the job of CSS. Or, it’s being emphasised, in which case there’s another element for that job already. Via: dev.w3.org.